Only one set of railroad tracks on a fragile sandstone bluff in Del Mar connects San Diego with all points north and south.

Multiple landslides on that bluff in the past year forced officials to temporarily halt trains for safety inspections on the nation’s second-busiest rail line.

But what would happen if the cliffs were to suffer a major collapse, damaging some or all of the track and putting the rail line out of commission for months?

The results, officials say, would be costly and far-reaching, touching everything from transportation to commerce to tourism to telecommunications to air quality. Even the nation’s military readiness.

“This is not something that you can allow to happen,” said Matt Tucker, executive director of North County Transit District, the agency that owns the tracks, operates the Coaster commuter trains, and leases the rights to freight trains and Amtrak passenger trains that run on the route.

Bluff collapses increased significantly last August after a year of relative quiet. Slides were reported Aug. 22, Sept. 27, and Oct. 5. Then, on Dec. 10, a 30-foot-wide chunk sloughed off between Ninth and 10th Streets, halting trains for two hours. Safety inspections cleared the way, but the December incident brought new focus to the erosion problem.

Studies show the coastal cliff erodes at an average rate of six inches annually. But by December, some spots in Del Mar had shrunk by six feet or more in the previous year. Still, between Ninth and 10th streets, the bluff had 30 feet or more between the tracks and the edge of the cliff. Other nearby sections have less space, but are protected by piles or walls.

Erosion took another big chunk Feb. 2, when a Del Mar bluff collapse halted train traffic between Sorrento Valley and Solana Beach for five hours.

Then, on Feb. 15, one of the biggest collapses occurred when a 55-foot-wide section peeled away in pieces about noon. A Scripps Institution of Oceanography technologist on the beach recorded the slide on video, which was widely seen on social media and the evening news.

The series of slides underscored the need to stabilize the bluffs, Tucker said.

“If we reached a critical point, we would de-fund all other capital projects to fund this,” Tucker said, noting, “We have more than $1 billion of capital needs.”

Without that stretch of Del Mar track, thousands more cars and hundreds of trucks would spill onto the already congested Interstate 5 every day, according to an analysis by the San Diego Association of Governments, the region’s planning agency.

Shipping costs would jump for vital imports such as new vehicles, construction materials and petroleum products. And tourists who travel by passenger train would go elsewhere.

Regional and state officials have been scrambling for years to prevent such a disaster by shoring up the scenic stretch of rail.

Three rounds of bluff stabilization projects have been completed at Del Mar since 1998 at a total cost of about $5 million, according to SANDAG. Those efforts included the installation of about 200 concrete-and-steel columns called “soldier piles.”

Each pile is 3 feet in diameter and goes as deep as 65 feet into the ground on the west side of the railroad. Together, the piles form a sort of retaining wall to protect the tracks from erosion.

The next phase of construction, with more piles and drainage structures, is expected to cost $3 million. It has been funded and is scheduled to start in the fall. State Sen. Toni Atkins announced in June the latest allocation of $6.1 million to plan and design more piles, seawalls, drains and anchoring devices.

Protecting the bluff-top route over the next 30 years may cost an additional $90 million in today’s dollars, according to SANDAG.

Eventually, erosion will force the route to be moved. SANDAG recently outlined preliminary plans that could cost $3 billion or more to relocate five miles of track inland, with nearly two miles of that traveling through tunnels.

That solution is probably decades away, but some elected officials have said it’s time to start working on it.

“Maintaining our heavy rail is essential for our regional economy,” said Assemblyman Todd Gloria, D-San Diego. “This is one of our region’s highest priorities, and we must choose a plan as quickly as possible so that we can begin to identify funding sources.”

A cost-benefit analysis SANDAG prepared for the construction to begin this fall looked at the economic consequences over six and 12 months of a significant bluff failure. It showed the effects would be felt across the region and beyond.

Repairs to the railroad after a 400-foot-long bluff collapse would cost about $40 million, the analysis showed. The cost of providing commuter train travelers with alternative transportation by bus for six months during the work would be an additional $2.4 million. The increased costs for shipping freight by truck during that time would be $109 million.

In all, the total economic costs of a six-month rail closure are projected to be $173 million. For 12 months, it would be $310 million.

The estimate does not include the costs to businesses that depend on the railroad to attract or sustain business. Nor does it account for probable tourism losses, though the report said the losses would be “substantial considering the importance of tourism in the San Diego region.”

It’s also difficult to assess how the railroad affects property values in Del Mar, where homes are among the most expensive in San Diego County. The city’s median home value is $2.5 million, according to the on-line real estate service Zillow, and homes on the bluff near the tracks are worth two or three times that amount.

If anything, the high property values make it unlikely that railroad could expand its right-of-way on the bluff.

Critical Cable

A large bluff collapse also could disrupt regional telecommunications.

Fiber-optic cables buried in the NCTD right-of-way parallel to the tracks could be severed, and the economic impact of that is expected to be severe, the SANDAG analysis states.

Some of the cables are owned by utilities including telecommunications companies such as Verizon. The transit district’s cables carry signals essential to “positive train control,” the federally mandated safety system that covers 60,000 miles of track across the United States.

The cables also link each of the Coaster commuter train’s eight passenger stations and carry information vital to fare collection, security and safety cameras, digital signs and other systems operated by the transit district, said NCTD spokeswoman Kimy Wall.

“This information is vital to NCTD in order to continue to run our commuter rail services and support interstate commerce,” Wall said.

Port of San Diego

Car sales are another element of the economy that depends on the stability of the Del Mar bluff.

One in every 10 new imported automobiles sold in the United States arrives by ship at the Port of San Diego and then heads north by rail.

Last year about 400,000 vehicles, mostly from the major Japanese and Korean manufacturers, were unloaded at the National City Marine Terminal, a port official said.

Nearly all those vehicles were loaded onto specially designed railroad cars and hauled up the San Diego County coast. Eventually, they ended up on sales lots in cities from Los Angeles to Chicago and Kansas City.

The National City terminal has the capacity to park about 20,000 vehicles, according to port documents. The average “dwell” time is 13 days before a vehicle moves from a cargo ship to a rail car for the next leg of its journey. At least one freight train loaded with automobiles, usually with 40 or more rail cars, leaves the port almost every night.

Dozens of other commodities such as lumber and building supplies leave the San Diego port by rail.

Railroad is what keeps San Diego competitive as a port city, said Josefina Balistrieri of the port’s Maritime Department. Without access to rail transit, the freighters would sail right past and unload at Los Angeles or some other West Coast port, she said.

“The cargo will go where it gets the best service and the best rate,” Balistrieri said.

Congested freeways

Besides providing affordable cargo transportation, the railroad helps tamp down the number of cars and trucks on the freeways and the pollution those vehicles create.

Traffic on Interstate 5 in San Diego County now averages more than 200,000 vehicles daily and is projected to increase to 300,000 vehicles by 2030, according to SANDAG. Trucks make 10,000 daily trips on I-5.

As many as 2,500 more passenger cars and an additional 600 semi trucks would travel the already crowded I-5 daily without the coastal rail route, according to the regional agency.

The 351-mile corridor between San Diego and San Luis Obispo on the Central Coast, known as the LOSSAN corridor, is the second-busiest rail route in the United States. It is exceeded only by the rail line between the two densely populated areas of New York City and Washington D.C.

Each year, more than 2.8 million intercity passengers and 4.4 million commuter riders travel on the LOSSAN corridor, according to SANDAG. One of every nine Amtrak riders in the country uses the route.

Amtrak riders increase significantly on holidays and for special events, said Amtrak spokeswoman Olivia Irvin. One of the busiest weeks of the year for passenger trains comes during the annual Comic-Con convention in San Diego.

Last year, Amtrak’s Pacific Surfliner carried more than 44,000 passengers to destinations between San Luis Obispo and San Diego during the five days of Comi-Con, a 40 percent increase in average daily passengers.

Rail transit is less expensive than the highway for moving large quantities of freight, SANDAG statistics show. Most of the freight is transferred from ship to rail at the Port of San Diego’s two marine terminals, one at Tenth Avenue and the other at the National City waterfront.

Shippers of all types of materials on the San Diego segment of the rail corridor together would pay an additional $604,812 per day, or $221 million in a year, to move those things by truck.

Some rail cargo is too large to easily load onto a truck, such as the huge blades used to build wind turbines. More common rail shipments are construction supplies such as lumber and drywall, carloads of bulk materials such as gravel or asphalt, and tankers of petroleum products.

Also, the rail corridor is part of the Defense Department’s Strategic Rail Corridor Network, which requires that it be available to move troops and equipment during a national emergency.

Back-up plan

Anytime rail service stops, whether for maintenance, repairs or inspections, the transit district has a back-up transportation plan for passengers. That usually includes what’s called a “bus bridge,” which takes people by bus on local roads or the nearby freeway from the point where rail service stops to where it begins again.

The bus-bridge procedure is the same whether the closure lasts an hour, a day or a month, said Jim Linthecum, director of mobility management for SANDAG.

Coaster commuter trains carry an average of 5,000 passengers a day, Linthecum said. There are multiple stops along the route, but most of the passengers who go through Del Mar are headed to or from jobs and other destinations in downtown San Diego that would put them on I-5 if the rail route were closed.

Planners estimate that during an extended track outage, about half those train commuters would switch to buses and half would use their personal vehicles.

“That’s another 122 buses on Interstate 5 and another 2,500 cars,” Linthecum said.

Truck vs. train

An average of six freight trains a day also use the tracks, he said. The cargo carried by those trains is equal to the contents of about 180,000 trucks a year and most of that also would go on the freeway if no railroad were available.

About 7,800 semi trucks traveled I-5 daily at the state Route 52 intersection and about 9,400 trucks daily at state Route 78, according to state Department of Transportation statistics for 2015.

Freight trains are “vital for goods movement in general” and especially for the Port of San Diego, said Jessica Gonzales, a SANDAG spokeswoman.

The value of commodities carried by train can vary widely, from more than $500,000 per carload for new automobiles to only $600 per carload for construction materials such as sand or gravel.

Shipping costs recorded by SANDAG show a huge financial advantage in the use of railroad transportation.

The average cost to haul materials by truck is about $45 per ton, compared to $9.23 per ton by rail, according to the agency’s statistics for 2015.

A typical freight train on the corridor between San Diego and Los Angeles has 40 cars, each carrying 65 tons of freight, or a total of 2,600 tons of freight per train.

That’s a savings of $604,812 each day for shipments by rail instead of truck. Over a year that amounts to a savings of more than $220 million for shipments by rail, the statistics show.

NCTD contracts with passenger carrier Amtrak, the national freight carrier Burlington, Northern and Santa Fe, and the local freight hauler Pacific Sun Railroad to run those companies’ trains on the tracks.

Freight trains usually operate at night to stay out of the way of the increasing numbers of passenger and commuter carriers, said Lena Kent, director of public affairs for BNSF.

BNSF, the only interstate freight rail line in San Diego County, carries goods of all types but mostly automobiles, construction supplies and shipping containers loaded with myriad types of merchandise. The company does not discuss details about cargo for security and proprietary reasons.

The local hauler, Pacific Sun, is based at the Stuart Mesa switching yard on Camp Pendleton just north of Oceanside. PacSun, as it’s called, carries corn, soy, lumber, plastic pellets, beer, paints and items bound for recycling to destinations within San Diego County, according to the company’s website.

Coaster trains carried a weekday average of 4,915 passengers in the 2017-18 fiscal year, with a total ridership of 1.5 million passengers for the year, according to NCTD. Overall ridership has declined in recent years, but transit officials say that could change at any time, especially as freeways become more crowded and mass transit is made more convenient.

NCTD and Amtrak both plan to increase the frequency of daily trains on the coastal route in the years ahead, all which will be crossing the Del Mar bluff.

--Phil Diehl is a reporter for The San Diego Union-Tribune