Railfan communications
I have both Amateur (ham) and GMRS licenses and often carry a handheld radio (HT) when railfanning. There are no established frequencies for radio communications. But these are my suggestions if you're chasing in a group:
End of Train Device frequencies
Although I don't list these frequencies in my database it's not a bad idea to scan for the End of Train Device (EOT) also called a Flashing Rear End Device (FRED). These low power radios send a 'squawk' at regular intervals. An active EOT may tell you a train is nearby (typically within a mile) when no other radio traffic is heard.
Best scanner for railfanning?
You see this question all the time. What's the best Bang for Your Bucks Won't Break the Bank scanner for railfaning? No question, it's the Uniden BC125AT. Danny Harmon's review will tell you all you need to know.