Hey Dave, I have been planning a trip to see WW2 battelfields myself. I have looked into the Battle of the Bulge and have a different answers than the previous individuals. Viator.com has a day tour from Brussels, it is their tour #6179Bulge_SG. According to their web site there is only one offering of this tour in Sep and that is on the 15th. The cost is $206.18 per person and they'll pick you up at your hotel. I encourage you to look into this tour to see if it would meet your needs. I've never used Viator so I don't know anything about them.
Another option is going to Bastogne on your own and hire a local guide. I've been in contact with a gentleman by the name of Marcellin Destordeur (email@example.com), I forgot where I got his name from but it was a review and the person who wrote the review thought highly of him. I'm leaning towards using him myself. Another person I communicated with is Henri Mignon (firstname.lastname@example.org). Perhaps others have used these gentlemen and can provide us feedback.
As for getting to Bastogne, as one of the other replies indicated there is no train service to Bastogne is correct. However, you can take the train from Brussels to Libramont and the bus from Libramont to Bastogne. Go to the Belgium National Railroad site and enter Brussels to Bastogne and they'll provide both the train and bus schedule and cost; the trips from Brussels Central is right over 3 hours. Their web site is http://www.belgianrail.be/jp/sncb-nmbs-routeplanner/query.exe/en?ld=std&seqnr=1&ident=kd.06532111.1431042533&OK#focus. If this doesn't work Google Belgium National RR.
Please let me know how your trip goes and what tour guide you used and a review.
I hope to go myself in October; but I may have to wait until Spring 2016.