For scanning aka receiving purposes there's really no such thing as a best antenna for any and all purposes, it's just not possible and no such thing will ever exist. You can, however, choose an antenna that is best for you in your given situation and for your given needs. The issue is that most small roof/trunk vehicle mount antennas are designed to be used for specific purposes. That's not to say that you can't get a great antenna, however.
Since you mentioning you're a rail fan, and knowing that railroads here the US and Canada (see the note I put at the end of this post) use not only a specific frequency band but they have assigned frequencies as specific channels that makes things really easy. The band they're all located in is the 159.800 to 161.600 MHz VHF range, with some systems in the 450-460-470 MHz range (although in my experience I've never detected or heard them used in those UHF areas). You can find the list (if you don't already have it) of the channel assignments and frequencies here:
On Track On Line - Scanner Radio
Now, for an antenna you could get something designed for the 150 MHz range and be done with it, plain and simple, and that'll work for exactly what you need it to do. You could even get a company to custom manufacture one for you cut to precisely 160.700 MHz or so which puts it in the center of those RR frequencies and make it the best tuned antenna for that purpose. Because I'm going to assume you might use that 346XT to listen in on other things sometimes (an assumption so I could be wrong, as noted) then what I'd suggest is something like the very popular Laird or Larsen (two different companies) tri-band antennas which typically work fantastic in the 150-450-800 MHz ranges. The Larsen can be found here:
https://www.theantennafarm.com/catalog/larsen-nmo150-450-800-1050.html
and the Laird can be found here:
http://www.theantennafarm.com/catalog/laird-technologies-abscanc-1048.html
They're both $29.95 each so the price is the same either way. Both of them get great reviews and discussion here at RR and some other sites, and also as noted they're very popular and perform well for many people. Either of them would certainly provide what you're looking for with excellent coverage in the VHF band where those railroad comms are going to take place plus they offer the capability of good/great coverage in the higher UHF as well as 800-900 MHz ranges just in case you later on decide "Hey, I have a great scanner here so why limit it to just 2 MHz of monitoring 'cause there's a lot going on in the world around me..."
Because both of those use the NMO (New Motorola mount) style you'll need to get an NMO magnetic mount for whichever antenna you end up with (of course you may choose something else entirely that I didn't recommend but some magnetic ones are here:
http://www.theantennafarm.com/catal...netic-mounts-265/nmo-magnet-mounts-other-279/
(note I'm not a customer of that website, it's just one that comes up when I do a search for the Laird/Larsen antennas - you may find the same antennas someplace else for lower pricing and NMO magnetic mounts as well, just providing the info for you to work with)
Just realized you're in Canada but it appears Canada honors the AAR channel and frequency assignments so you should be good to go. Might be some Canadian antenna retailers you can look into as well, or hit up eBay's Canadian site and find one of those antennas as well as a magnetic mount and you should be good to go.
Happy holidays.
