WASHINGTON — Senate Democrats on Thursday demanded that Republicans provide more money for Amtrak so the railroad can tackle a $21 billion maintenance and repair backlog, including replacing tunnels more than a century old.

Democrats at a news conference cited last week’s deadly crash in Philadelphia and said the backlog is compromising safety and service.

Investigators have said the crash could have been prevented if expensive safety technology called positive train control had been in operation. The technology can prevent derailments due to excessive speed and collisions between trains. Amtrak says the system will be ready by year’s end throughout the Northeast Corridor, which stretches from Boston to Washington, with the exception of some track owned by commuter railroads in New York.

Democrats want Congress to give Amtrak the entire $2 billion in subsidies the railroad requested for the budget year that starts Oct. 1. They say GOP lawmakers have starved the railroad financially for years.

A Republican-controlled House panel approved a spending bill the day after the crash that provides Amtrak with $1.1 billion, a cut of $251 million from this year.

A spokesman for Senate Majority Leader Mitch McConnell, R-Ky., didn’t immediately respond to a request for comment.

Republicans have long criticized the nation’s long-distance passenger railroad as wasteful and inefficient. Outside the busy Northeast Corridor, Amtrak service generally doesn’t turn a profit.

Amtrak more heavily serves urban areas in the Northeast, Midwest and West Coast that are predominantly served by Democrats than less populated, GOP-leaning states in the South and West.

The budget that President Barack Obama submitted to Congress in February proposed allotting Amtrak $2.5 billion next year, with most of the increase dedicated to capital investment in tracks, tunnels and bridge.

“Throughout the Northeast Corridor, Amtrak has some infrastructure that is so old, it was built and put into service when Jesse James and Butch Cassidy were still alive and robbing trains,” said Senator Chuck Schumer, D-N.Y.

One tunnel in Maryland that needs replacement is more than 140 years old. Tunnels under the Hudson River between New York and New Jersey are over 100 years old.

“We should not be putting Amtrak in a position of having to choose between safety, like positive train control and other technologies, and investing in fixing crumbling bridges or crumbling infrastructure,” said Sen. Bob Casey, D-Pa.