The main action sequence of the episode of NCIS: Los Angles that first aired on November 5, 2013 featured a train – one engine and two cars each containing two tanks of chlorine gas. As a moderate rail-fan with an incomplete knowledge of rail details, I think I spotted at least three major flaws – two of which might have (ahem.) derailed the plot if I’d let them.
First: The train had been hijacked by a recently fired brakeman, who was otherwise about to be promoted to engineer. He was taking action because the railroad owners were covering up that they were routing dangerous cargo through residential areas. His protest was to take the train and drive it to LA Union Station.
But, I’m pretty sure that the central control of the switches has as much, or more, to do with where a train ends up. So, his plans to get this train to Union Station were pretty much shot from the moment it became known that the train wasn’t properly manned.
Second: It turned out that the hijacker had been duped by a couple of others who instead planned on setting off explosives under the tracks, derailing the train and causing the chlorine gas tanks to break open, injuring and killing a lot of people in or near a major metropolitan area.
When the hijacker learned this, he attempted to stop the train. However, the brake line between the engine and the first car failed catastrophically, leaving him without breaks. As I understand it, modern train brakes are still based on the old Westinghouse Brake – namely that they are held disengaged by the air pressure in the system. A break in the brake line that caused the air to pour out (as shown in the episode) should have caused the brakes on the two cars to engage. As long as the brakes on all of the trucks engaged at about the same time, or the brakes engaged back to front, the cars would have simply decoupled from the engine and stopped. If the front car’s brakes engaged first, the rear car could have derailed.
Third: Once the hijacker learned that the brakes weren’t working and that he couldn’t decouple the train while moving, he “reversed the polarity” of the engine to slow the train that way. Now, as I understand it, the primary brake on a diesel-electric engine is to drop a big resistor across the wheel motors (and use the fans on the top of the engine to dump the resulting heat). So this wasn’t that far off what he would have already been doing.
But, this somehow locked the wheels not only on the engine, but also both cars, resulting in them throwing sparks from the friction between the wheels and the rails (done, no doubt, by the visual effects crew). Now, I think a dead-short on the wheel motors would cause them to lock (or come pretty darn close), which would cause sparks there. But the wheels on the cars should have been still free (after all, their brakes didn’t work).
Finally, I suspect that locking the wheels on the engine without brakes on the cars would have just about assured that one or both of the rear cars would derail – the exact thing that they didn’t want to happen.
Now, they were right with the amount of distance a train at speed – even a fairly light train like they showed – would take to stop. But I think this was as much to create tension when one of the main characters was trying to defuse the bomb with seconds before the train tripped it.