In my state, train companies are fined if they block a crossing more than 10 minutes during the day or 15 minutes at night. But every so often we have to remind local politicians that those regs need to be upheld.

To answer your question directly, Diesel locomotives are actually Diesel generators that drive electric motors. When they’re stopped they cycle down the generators to a minimal count to maintain power. They’re more efficient in general because the engines are generally running at their peak efficiency RPMs (to drive gennies instead of the wheels). Unlike trucks that are rarely running at peak efficiency RPMs and will burn extra dirty at idle.