Like Danny Harmon's video mentions, a 1/4 wave whip tuned near the rail frequencies on your car roof is as good as it gets, plus it does decent on the UHF EOTD and DPU frequencies. That's important in dark territory, anywhere train crews don't have to call signals, or there aren't talking detectors. It's an age-old argument, but at least for monitoring I've never noticed a huge difference 1/4 vs. 1/2 vs. 5/8 wave for voice.
Smiley makes a telescoping 1/2 wave for the 2m ham band that you can shorten to be closer to the rail channels that does better than a rubber duck since it doesn't need a ground plane, but it's a bit awkward having a 3' antenna on a handheld.
Smiley makes a telescoping 1/2 wave for the 2m ham band that you can shorten to be closer to the rail channels that does better than a rubber duck since it doesn't need a ground plane, but it's a bit awkward having a 3' antenna on a handheld.