On the main Vienna-Salzburg line
St Pölten is on the Westbahn line - between Vienna and Salzburg. If you are travelling from Vienna, then you can reach St Pöten in 40 minutes on the fastest InterCity trains - non-stop - such as the train to Zurich and/or all services to Salzburg and Munich. Otherwise there are also the slower Regionalzug (regional train services - their numbers are prefixed by an R) services, which take an hour and twenty minutes for the same trip, but stop far more frequently. If you are travelling from Vienna, then you might like to purchase your ticket from the stop Purkersdorf Sanatorium, if you have a valid Vienna travel pass - you save some money by doing so.
Travelling from Salzburg takes about 2 hours and 35 minutes, unless you jump on an ICE service which does the journey in a whisker over 2 hours - but hey, what do you do with an extra half hour in St Pölten?
In St. Pölten you can find a lot of very nice bicycle paths. You can rent a bike at the train-station and bike around the whole town easily. You'll see, it's very nice, and you won't miss any of the must-sees!
You can easily reach my hometown by train from Vienna in about 3/4 of an hour. You'll arrive at our main station (picture) and find yourself here directly in the City Center.