Railroad museums and scenic railroads have some beautiful T-shirts in their gift shops and online, but all for prototype railroads, not model trains. The only shirts I could imagine anybody wanting in the toy/model train field would feature the classic Lionel catalog covers of the pre- and postwar periods.
Speaking for myself, there is zero point zero possibility that I would buy or wear a purple shirt with an MTH logo, or Lionel orange and blue. I do have a polo shirt with a nice conservative Weaver logo, but I ONLY wear it to train shows.