Or just keep trains moving. I'm shocked that US freight trains are left parked for so long in areas known to be theft-prone with poor fencing. I don't think these kinds of thieves would risk their lives going after a train running full speed.
How do truck shippers protect their wares? 1 driver per truck with eyes on the load all the way through. If they have to take a nap, it's in the truck...
How do valuables travel? In armored cars with armed personnel.
The train company isn't entitled to its business model of very long trains with loose security.