SGerding
Hello & Welcome to the Forum!

I'm sorry to hear of your locomotive problem.
I suspect it is NOT a problem with your transformer.
This is a fairy common problem that is usually caused
by a bad/dirty motor/armature.

Here's a link to:
Maintaining Your Lionel Train
www.slsprr.net/technical/PopSci%20RefLibr%20No.15.pdf
You will want to follow the the instructions: How to Clean Lionel Motors

You might also want to familiarize yourself with the following:
Common Repairs for Electric Trains
http://www.slsprr.net/technical/PopSci%20RefLibr%20No.15.pdf

You will also want to clean the locomotive wheels & pickup rollers, then lube as suggested.
If after checking the transformer & doing the cleaning and maintenance to the motor your locomotive still slows/stops,
you will need to check out the e unit/reversing unit for proper operation.

Happy Rails!
Dave