How much will Philadelphia crash cost Amtrak?

Officials meet at the site of a derailed Amtrak train in Philadelphia, May 13, 2015.
Mike Segar | Reuters
Officials meet at the site of a derailed Amtrak train in Philadelphia, May 13, 2015.

As victims of the Philadelphia derailment begin filing lawsuits against Amtrak, the railroad could face damages of as much as half a billion dollars. (Tweet This)

That's how much a new bill in Congress, lifting a nearly 20-year-old liability cap, would raise the combined total paid to victims and their families to collect for crash-related losses.

It's too early to estimate the final tally for legal claims in the May 12 accident, which killed eight people and injured more than 200 after a northbound Amtrak train traveling at more the twice the speed limit hit a curve and jumped the tracks. But the amount could well exceed a 1997 limit that caps claims related to a single accident to $200 million.

Here's how the process will likely unfold.

Why is the amount limited in the first place?

The limit was put in place as part of a rescue for the national passenger rail service that was designed to put it on a more solid financial footing. At the time, Amtrak's biggest accident settlement totaled $58 million, in a case involving a January 1987 collision with a Conrail train outside Baltimore that killed 16 people.

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But the cap wasn't adjusted for inflation, which in today's dollars would amount to almost $300 million. And it looks like the current $200 million cap "may not be enough for medical and other expenses given the scope of the potential damages in the May 12 Amtrak crash," according to a statement from Sen. Bill Nelson, D-Florida, who has introduced a bill to lift the cap to $500 million.

Has this ever happened?

Not to Amtrak, but the cap was a factor in a 2008 crash in Southern California, when a Metrolink commuter train collided with a freight train, killing 25 people and injuring more than 100. In that case, the combined claims topped the $200 million limit, so the case was consolidated in federal court, where a judge doled out partial payments to each claimant.

The cap doesn't apply to Amtrak employees, who are eligible for workmen's' compensation for injuries suffered on the job. An Amtrak employee filed the first lawsuit last week, asking for more than $150,000 in damages.

What if it turns out the crash wasn't the engineer's fault?

That question has yet to be settled, but it could come into play depending on the final determination by safety investigators. Over the weekend, they reported that a rock or other heavy object may have been thrown at the locomotive's windshield just before the crash. That could have stunned the engineer and prevented him from slowing the train, which was moving at 106 mph, more than twice the speed allowed, when it derailed on a tight curve shortly after leaving Philadelphia's 30th Street Station.

Read MoreFeds not sure if Amtrak windshield was hit

On Saturday, federal officials ordered Amtrak to install the system on the northbound side of the curve. The system, known as automatic train control, alerts an engineer to excessive speed and automatically applies the brakes if the train doesn't slow down. It is already installed on the southbound side of the curve.

Amtrak also faces a year-end mandate to install a much more sophisticated and far more expensive technology known as positive train control across its entire rail network. The simpler automatic train control system, though, has been in place on dangerous curves since the 1990s.

Even if the engineer isn't found responsible, Amtrak should be held liable for not installing automatic speed control on the curve where the accident happened, according to George Cahill, a Connecticut attorney representing one of the victims.

Read MoreUnions urge Amtrak to put 2nd engineer in every locomotive

"No matter what they find out happened to the engineer, Amtrak should be responsible for the negligence in not protecting the (northbound) curve with automatic train control," he said. "It's a very small cost to put a circuit in and activate it."

Amtrak resumes service on the Northeast corridor
Amtrak resumes service on the Northeast corridor   

Who has to pay the victims once all their claims have been filed?

Amtrak ultimately has to pick up the cost. It's not clear whether the railroad is covered by insurance, or whether the money would have to come from already limited funds. An Amtrak spokesman was not immediately available.

But, if the cap is lifted, Amtrak can ill-afford shelling out $500 million. On the day after the Philadelphia crash, House Republicans voted to cut $251 million from Amtrak's budget next year, to $1.1 billion. The appropriations panel also nixed a Democratic bid to boost Amtrak spending by more than $1 billion, including new funds for the busy Northeast corridor.