An Amtrak train derailed in Philadelphia on Tuesday night, killing at least seven people and injuring more than 140 others. Federal investigators are still working to determine what caused the derailment, which halted some service in Amtrak’s highly-trafficked Northeast Corridor.

The images and stories coming from the scene could raise a valid question for unsettled travelers: How often does this sort of thing happen? The answer: Every so often, but not as frequently as it used to.

Examining derailments over four decades

Amtrak started service in May 1971 with a train traveling between Philadelphia and New York, but it wasn’t until 1976 that the rail agency took over service on the Northeast Corridor, spanning D.C. to Boston. This stretch would become vital to Amtrak’s operations and is now the busiest passenger rail line in the country, according to the rail agency. In 2013, Amtrak reported that 11.6 million riders traveled in the Northeast, representing more than a third of all Amtrak ridership nationwide.

Over the past decade, there have been about 31 derailments per year, according to the Federal Railroad Administration. That number is significantly down from the 54 derailments that occurred per year over the decade before, a span that included two years — 2000 and 2001 — with 156 combined derailments, the most over any two-year period since the mid-1970s.

Still, even as the number of derailments has gone down, the number of passengers injured on Amtrak trains has risen in recent years, as my colleague Chris Ingraham documents here. Passenger deaths are still pretty rare, Ingraham notes.

Trains are generally considered a safe mode of transportation. Roadway deaths make up nearly 94 percent of all transportation-related fatalities in the United States, and rail-related deaths are much less common, according to the National Transportation Safety Board.

In 2013, there were more than 32,700 deaths on a roadway, compared to 891 railroad deaths, the NTSB says. Most of these rail-related deaths involved people trespassing on the tracks who were struck and killed by trains. There are also accidents like the fatal commuter train crash near Manhattan earlier this year, when a Metro-North train smashed into an SUV on the tracks and six people were killed.

Why Amtrak trains derail

The deadliest crash in Amtrak history took place in 1993, when a train traveling from Los Angeles to Miami derailed on a bridge near Mobile, Ala., and multiple rail cars fell into the water. This crash killed 47 people — 42 passengers and five crew members — and injured more than 100 others.

The NTSB determined that barges being pushed by a towboat in the early-morning hours hit the Big Bayou Canot railroad bridge, shifting the bridge enough that the train derailed a few minutes later.

Half a century earlier, one of the country’s worst rail disasters occurred not far from where the train derailed in Philadelphia on Tuesday. A train derailed on Labor Day 1943, killing 79 people and wounding more than 100 others.

When Amtrak trains derail, investigators can point to a variety of factors. Rail conditions come up often: A derailment in Washington state in 2005 was blamed on the track, as was a derailment in Mississippi a year earlier that killed one person. (Many people quickly raised the issue of Amtrak’s crumbling infrastructure after the derailment on Tuesday — the rail agency has cited the “decay” on its Northeast Corridor — but investigators say they haven’t identified a cause yet.)

Just a month after the rail agency started service, a train in Illinois derailed because of what the NTSB said was a rail being moved by a part of the train; 11 people were killed. Another derailment in 1976 was also blamed on the rail moving because of the train’s movement.

In 2002, Amtrak’s Auto Train derailed near Crescent City, Fla., killing four people; the NTSB also pointed to track problems based on improper track maintenance. That occurred a year after a train in Iowa derailed, killing one person, and investigators again found rail failure.

An Iowa derailment in 1990 said that the rail had been improperly installed, while 1997 derailment in Arizona was blamed on erosion that moved the track. A fatal Vermont crash in 1984 occurred because a flash flood destroyed the embankment supporting the railroad, the NTSB said, and five people ultimately died. Meanwhile, in 1979, a train derailed in Kansas because an engineer was driving too quickly, as was a derailment in Illinois the following year.

Sometimes, investigators point to multiple factors. After an Amtrak train derailed in Texas in 1998, the NTSB listed a host of explanations, saying that the track conditions were bad, a dispatcher waited too long to pass that information along, the train’s engineer did the same thing and the Union Pacific Railroad didn’t properly oversee track maintenance.