My grandparents took me to the Strasburg Railroad often as a child and I just recently took my daughter. It's a beautiful railroad in the Amish countryside that offers a train ride (can be a little pricey depending on what car you want to ride in) as well as some nice little shops and a cafe. The set up is almost like a little village beside the train. The train ride is wonderful and very child-friendly! If you decide to go during the winter, call ahead because they usually close down for about two months (this year it was January 1st through President's Day). It is a place your children will definitely enjoy!
I have a 4 year old who is absolutely in love with trains! On the day we rode this train, he wore his conductor outfit. This train is very easy to find, ample parking available, and is simple to get tickets for. I suggest calling the day or two before and purchase your tickets to make sure you are able to ride. Also, there are plenty of things to do at the train station that you will want to arrive early or plan to stay late. If you take the last train ride of the day, be advised that the stores will be closed when you get back to station. The narration is wonderful, even interesting for the children. We had a blast and would do it again!