Here is a rather technical site which explains why 5/8th wave antennas (and many other antennas) have more gain, but honestly, this is way toooooo technical for this forum.
I was unable to quickly find anyone in the U.S. selling 5/8th wave antennas specifically designed for the rail frequencies. I did find a dealer in England selling marine 5/8th wave antennas for boats (156-160 mHz) which use frequencies very close to railroads. These antennas started at about $200 US. You can also find lots of dealers selling 5/8th wave mobile antennas for hams (144-148 mHz), but these are too far away in frequency from 161 mHz to give great rail performance.
If you are determined to get "the BEST" antenna for mobile rail monitoring at any price, you should contact your local two-way radio dealers who could help you (for a price!).
Antennas with even more gain than 5/8th wave models also exist. One type is called a co-linear antenna. If you are interested, you can do a Google on these (and other) antennas to get all sorts of technical info.
But this is all getting way too technical for 99.9% of those using this site.
And, it boils down to how much money do you have?
IMHO, a properly cut (in length) quarter wave using quality coax cable will perform adequately in nearly all railfan cases.