it's to prevent a runaway or otherwise unwanted train from continuing down the track.
It sounded ridiculous at first but it makes sense. Rather than gain momentum and hit something important at the end of the track, better to derail it sooner. A big barrier in the middle wouldn't necessarily stop the cars, but a derailer certainly will.
And it may sound strange, but it's probably cheaper and less dangerous to knock cars off the track than to let them hit whatever is at the end of the track. Assuming they're going slow down the siding, you're probably just going to get one or two derailed and then the rest stop - assuming there are more than a few to start with.
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