As the massive seismic retrofit of the Transbay Tube gets underway, BART on Monday will begin subbing buses for trains in its first hour of service and single-tracking trains after 9 p.m. on weekdays.

The service changes will allow the agency to complete the more than $300 million retrofit of the Transbay Tube, shoring the essential transit artery in the event of a major, once-in-1,000-years earthquake, while also performing needed repairs and upgrades throughout the system.

It takes crews some time to set up equipment at night and then break it down before the start of the morning commute, creating a limited window of “wrench time” to actually get to work. Adding extra hours for workers will enable the agency to get 43 percent to 62 percent more work done and shave months off construction, officials said.

The remaining seismic retrofit work in the Transbay Tube is the last project in a host of earthquake upgrades voters approved in 2004 as part of $980 million in Measure AA bonds for the $1.2 billion program. Crews will line key sections of the 3.6-mile tube with curved steel plates, using grout to adhere the plates to the tube’s concrete walls, and a polymer product under the tracks to prevent against possible leaks. They’ll also install an upgraded pumping system that will allow larger quantities of water to be removed more quickly from the tube, should water make its way in.

Throughout the rest of the system, workers will replace segments of rail and upgrade power substations, rebuild interlocking components on the tracks and replace high-voltage transformers, upgrade fire and tunnel lighting systems, and replace stretches of its electric cables, among other projects. It’s largely work voters approved in 2016 under Measure RR, a $3.5 billion bond aimed at improving reliability on the aging system.

The work is expected to last three and a half years, but BART officials say the extra time spent working will save them time and money in the long-run, delivering the projects faster and improving passengers’ commutes.

Some 2,900 passengers board BART between 3:45 a.m. and 4:45 a.m., with the vast majority boarding in the East Bay and more than half disembarking in San Francisco. BART will run buses from select stations to destinations in the East Bay, Peninsula and San Francisco. Single-tracking after 9 p.m. through the tube mean trains will come every 24 minutes, rather than every 20 minutes, and some passengers will need to make an extra transfer to get to their destinations. There will be extra trains added on Friday nights.

What will that mean for you? See below for the best way to plan your commute.

What you need to know:

Carry cash or a pre-loaded Clipper card to board early morning buses. You won’t be able to use one of BART’s paper tickets on the early morning buses.

Download the BART (Official) app for Andriod of iOS or bookmark www.BART.Gov/Planner on your desktop to plan your trip.

It’d probably be helpful to use the app or desktop trip planner every time you ride, but it’ll be essential on Sundays. BART plans to upgrade its electrical power station in downtown San Francisco, meaning it will be single-tracking through the area on some Sundays. For the most part, only Antioch/Millbrae trains will provide service through the tube, but there might also be direct Daly City/Warm Springs and Daly City/Richmond trains, depending on when and where crews are working.

On Sunday, Dublin/Pleasanton trains will operate between Dublin/Pleasanton and MacArthur and all Millbrae riders must transfer to a San Francisco/Antioch train at SFO.

Early Bird Bus service:

When: 3:50 a.m. and 5:30 a.m.

3:50 a.m. and 5:30 a.m. Where: The Early Bird Bus service includes 15 new bus lines that will run in both directions between stops. There will be seven transbay bus routes, five for destinations in the East Bay, two for stops in San Francisco and the Peninsula, and one for Contra Costa County. The bus will serve 21 BART stations and the Temporary Transbay Terminal in San Francisco.

The Early Bird Bus service includes 15 new bus lines that will run in both directions between stops. There will be seven transbay bus routes, five for destinations in the East Bay, two for stops in San Francisco and the Peninsula, and one for Contra Costa County. The bus will serve 21 BART stations and the Temporary Transbay Terminal in San Francisco. How much: Fares will be as much or less than a BART ticket.

Fares will be as much or less than a BART ticket. How to pay: Cash or Clipper card. No paper BART tickets will be valid.

Cash or Clipper card. No paper BART tickets will be valid. For Golden Gate Transit passengers only: You must tag on and off, like on BART, or you will charged the maximum fare. For all other operators, passengers need only to tag on.

You must tag on and off, like on BART, or you will charged the maximum fare. For all other operators, passengers need only to tag on. Parking: You can continue to use BART’s lots to park, but you must pay for parking on the BART (Official) app. You won’t be able to enter the station and pay for parking like you do now. If you don’t have a smartphone, you can also visit www.Select-A-Spot.com/BART on your desktop to pay for a parking spot. You’ll have to print out the tag and place it on your dash. Or, if you want to pay in cash, you can purchase one-day hang tags in person at the customer service centers at Lake Merritt or Embarcadero. Scratch the date you want and hang it on your rear-view mirror or place it on your dashboard. Always park in the fee area of the lot.

You can continue to use BART’s lots to park, but you must pay for parking on the BART (Official) app. You won’t be able to enter the station and pay for parking like you do now. If you don’t have a smartphone, you can also visit www.Select-A-Spot.com/BART on your desktop to pay for a parking spot. You’ll have to print out the tag and place it on your dash. Or, if you want to pay in cash, you can purchase one-day hang tags in person at the customer service centers at Lake Merritt or Embarcadero. Scratch the date you want and hang it on your rear-view mirror or place it on your dashboard. Always park in the fee area of the lot. More information: Detailed information on specific stations can be found at www.BART.Gov/Schedules/Early-Bird-Express.

Start of service: