I believe I still have a Union Pacific themed mug with the California Zephyr crossing the Rocky Mountains at some point. There may be others lurking about.
When I travelled by train from Toronto to Vancouver, and then from Seattle to Los Angeles, I was given, and/or won or bought several lightweight items like cap badges, and key rings and coasters, maps and similar items. I did not think that a mug was likely to make it one piece over the Atlantic.
My host in Los Angeles (Orange County) gave me cotton quilt with American steam trains on it. This needed a bag of its own to be transported and caused a great deal of concern when Canadian Customs thought I had three items of luggage for a week's stay returning to Canada from the States.
My souvenir mugs were bought mostly in Europe when I was travelling back to the UK by train. They were easy to pack in a tee shirt and carry in my hand luggage.