Go to Menards.com and in the search field type in these words: train stuff.
Then, scroll through the current product listings to see what you think the boy might like. Open hopper cars are often popular with younger children because they like to put stuff inside. Boxcars with colorful schemes, such as the M&M cars just released, are a safe bet.
If it is for an older child, you’ll have to assess his interests to make your choices.