Train Sounds
You can buy him the sounds of a diesel or classic steam locomotive on CD that are from the 1940s and 1950s for Valentine's Day. While he's tinkering with his train set, he can put on a CD and provide himself with hours of fun and think of you lovingly while listening.
Model Train Diploma
Another great Valentine's Day gift for a model train enthusiast is a diploma from the Model Trains University, which will put his name on the diploma, affix a gold seal and package it especially for him. What's more, you can frame it so he can hang the diploma on his wall. If you prefer to make your own, any number of computer programs will provide you with everything, including a fitting border. Either way, he will be tickled to receive it on Valentine's Day, and he will think of you whenever he looks at it.
Railroad Car
Buy your man a railroad car to add to his collection of model trains for Valentine's Day, but only after you do a bit of homework. You must find out the scale of his train set and if he prefers a particular manufacturer so you can give him something that he appreciates and will fit in with his collection. Head off to your local hobby shop, model train store, or to any one of several websites that have model railroad cars for sale.
Train Books
From the history of model trains to ideas on how best to expand one, a book about model trains will keep your man occupied for hours after you give it to him on Valentine's Day. For example, there are how-to books on simulating a real background, books on maintenance of what he has already purchased, and many more.
Landscaping
A model train enthusiast is always looking for ways to improve the landscape that surrounds his set of trains. For instance, he may have his heart set on a bridge trestle or a water tower, among other improvements. Buy him one of those items for Valentine's Day and impress him with your kindness. Just be sure that it fits with the other elements of his landscape.