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

The Lionel #8902 locomotive is a DC locomotive, and needs a DC power supply.

Lionel has vintage DC controllers such as this.
https://www.ebay.com/c/1724622794

A more powerful modern unit such as this one can also be used if you plan on expanding your empire with DC locomotives.
https://www.amazon.com/Model-Rectifier-Corporation-AMPAC-Controller/dp/B00CTMBM6I/ref=sr_1_2?dchild=1&keywords=LGB+Train+Transformer&qid=1601353660&sr=8-2

If you want to expand your empire, you will be limited in the Lionel engines you can run with DC power, as most run on AC power. You would have to disconnect any whistle or horn relays on AC powered locos, as they are triggered by DC.

It would be recommended that you take your loco to a Lionel service station or a local electronics repair person, and request that a full wave bridge rectifier be installed. This way the loco will run with any Lionel AC transformer. It would run in only one direction with no reverse. If you want reverse capability, an electronic reverse unit would need to be installed instead, such as this one.
https://www.ebay.com/itm/Lionel-Parts-610-8719-103-electronic-E-unit-board/333725834824?hash=item4db39c1e48:g:U9kAAOSwsdtd-8kO

Larry