Ever since the deadly derailment of Amtrak’s Northeast Regional Train 188 in Philadelphia, some members of Congress have demanded to know why the nation’s railroads haven’t finished installing the high-tech equipment that might have prevented the crash.

They know why.

It’s because Congress ordered the railroads to install a complex and expensive technology and gave them a deadline, but no money. Now, the Federal Railroad Administration says it will fine the railroads $25,000 a day if they don’t have the equipment in place by the end of this year.

The system, called positive train control, is meant to backstop human error. It uses GPS, wireless onboard radio and other components to detect an imminent crash and override the actions of the engineer.

Amtrak’s Philadelphia-to-Washington train was going more than 100 mph as it approached a 50 mph curve minutes after leaving Philadelphia’s 30th Street Station on May 12. Eight people were killed and more than 200 injured. Safety officials say the accident could have been avoided if the stretch of track was equipped with PTC.

In Illinois, they’ve pointed to derailments on Metra commuter lines, in 2003 and 2005.

A head-on collision between a Metrolink train and a freight engine in Los Angeles killed 25 people in 2008, prompting Congress to order passenger and freight railroads to install the system by the end of 2015.

It’s been clear for a long time that the railroads won’t make that deadline, and not because they’re not trying. It’s in their interest to install the system — not just to prevent the relatively rare accidents involving passengers, but to minimize the routine but costly derailments in which a load of corn gets spilled or a track is taken out of service.

The railroads have spent close to $7 billion on PTC so far. The cost to deploy the system nationwide is estimated at $13 billion.

Metra originally pegged its cost at $400 million but has since been able to bring it down to $350 million. Installing the system is complicated, because Metra doesn’t own all the tracks it uses and shares some of the ones it does. The equipment has to work seamlessly for all lines that use the same tracks. Metra expects to have PTC fully running by 2019.

CSX Transportation, one of the nation’s largest freight railroads, has spent $1.2 billion so far. It doesn’t expect to complete work on the system until the end of 2020.

Only about 30 percent of the nation’s commuter rail systems — and none of the major freight railroads — will meet the December 2015 deadline, federal regulators said last week. That update came at a House subcommittee meeting at which railroad officials, including Metra’s Executive Director Don Orseno, made a case for more flexibility.

Not long ago, it looked like Congress would offer some relief. A bill in the Senate would grant a blanket five-year extension to install PTC. But enthusiasm for that measure fell off after the Amtrak crash.

The Obama administration, meanwhile, has asked Congress repeatedly for the authority to give railroads more time, on a case-by-case basis, as long as they’re making credible progress. No dice there, either.

That leaves the railroad administration no choice but to impose those $25,000-a-day fines starting Jan. 1, its acting chief, Sarah Feinberg, told the subcommittee. That makes zero sense. It would not make the impossible possible. All it would do is take even more money away from projects that were already shoved down the priority list by PTC.

We’re not convinced positive train control is a good investment to begin with. More lives would probably be saved if those billions were spent to install gates, signals and other safety features at railroad crossings, where most train accidents occur. Or to maintain tracks, which would prevent most derailments.

But railroads have had no choice, and they’ve been scrambling to meet that unrealistic deadline. Congress needs to cut them some slack.

“It doesn’t matter how big the bear is running after you,” CSX Vice President Frank Lonegro told the subcommittee last week. “We can’t run any faster.