Because there's no possibility that shooting at a train would result in a fireball a thousand yards high, whereas...

How thick are propane tanker cars?

Back of the bar napkin calculation with Barlow's formula, assuming 200psi (about what bbq tanks are at iirc?), somewhere around half an inch.

Bored people shoot track, put pennies on tracks, drop water balloons from freeway overpasses, put potatoes in car exhaust pipes, and on and on. Its important to consider the most random variable in the safety equation - bored people.

I'd say all these concerns warrant putting the damn thing underground.

Are you sure? I imagine most trains these days are pretty tolerant of that, it could easily go unnoticed.

Because the tube isn't a train?