The posters are up for Fullerton Railroad Days. This is a terrific event that takes place at the Fullerton Railroad Station (near the Spaghetti Factory, if you know where that is). This year it's May 3rd & 4th, from 9-5.
Oh, and did I mention it's FREE?
Disneyland will be sending the Fred Gurley steam engine, which, if like the last couple years, will be available for people to climb inside and have an engineer explain the controls to you, as well as pose for photographs.
There are usually several train cars/engines available to walk through, etc. Also, and this is one of my favorite things - several model railroading groups set up massive model railroads for you to check out. One of the most stunning, IMHO, is the Lego group.
Here are a couple images from last year: