Welcome to the forum!
Transformers have voltage outputs.
What Lionel locomotive do you have, and what transformer do you have?
It sounds like you have a regular transformer with one output, and not a transformer for Lionel trains. A Lionel transformer has a variable output, allowing you to control the speed.
You need a Lionel transformer and the necessary adapter to plug into your electrical outlets. Good, used Lionel transformers and other trains and equipment can be found on the online auction sites.
Larry