Hello David, and welcome...
What a trunk scanner is one that follows various government agencies and businesses on a "trunked system", which in short makes use of a few frequencies by many users. If all you want to do is casual monitoring of railroad operations, then you do NOT need a trunk scanner.
Look around besides Radio Shack, and don't spend alot of money. You want a handheld scanner (something like a handy-talkie) that monitors 160-161 MHz in the FM mode. That's it. You should be able to dig one up for less than a $100 that can continuously tune that frequency (as opposed to a radio with a few channels). Marine frequencies are just below that and aviation freqs are AM mode 118 - 136 MHz. Ask around other lists for someone looking to part with a used one. It should have a good battery (or take regular batteries), a charger and an antenna. A car power cord is handy, as is a magnetic mount antenna with coaxial cable to connect to your radio.
I have an Icom radio that is the size of a pack of cigarettes, tunes 30-1300 MHz and takes AA batteries. It wasn't much and works great for me. You can program your favorite frequencies and it will scan for them. A place to look for frequencies is www.fordyce.org
and click on the CT tab.
A word to the wise - some states it is not permissible to monitor communications and you can get your chops busted by the police. This is true in this day and age, and if they see a railfan with a scanner near a crossing, they can give you hell for it even if you're on public property. While this may seem harsh and big brotherish, they can do it LEGALLY. The one exception is if you hold a ham radio license (at least in NY) and if you want more info look at www.arrl.org
- one of the folks that work for the ARRL is a regular on the NHRHTA list, so ask there...
Best regards and good luck,
Jim (former NHRHTA'er)