Well before the fanboys chime in - both Android and Blackberry phones can tether, the issue is really what else you expect your phone to do for you. Will you be a heavy email user? Web, apps, games? If you want good quality advice, its a good idea if you post more info on what your specific needs are so people here can provide some recommendations.
FWIW, I've tethered extensively on Metrolink in Southern California with both Sprint (EVDO) and Verizon (3G) and I think you will find the performance to be "hit and miss". Our trains down here get up to about 90mph during rush hour, and even the best phones are going to have a hell of a time keeping a reliable connection open. The issue isn't the phone, the reception, or the network quality - its the speed at which your data connection must be transferred from one tower to the next. As you move along the tracks, each tower is in range for only a few seconds, meaning your phone (and the cell network) is having a hell of a time keeping your connection open as you zip along the tracks.
For browsing websites and firing off emails you might not notice any issues at all. But if you are trying to keep persistent connections open like SSH, FTP, SIP/VOIP etc. or doing remote desktop stuff - expect to run into lots of dropped connections on your commute.
Maybe WiMax or 4G/LTE is a better alternative for this type of situation - I haven't tried it but if you make the jump, please reply back and let us know how you like it.
Good luck!
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