My Dad and 10yr old son are both Train fanactics. We go to Interbay train yard on the south side of the Ballard Bridge for train watching lots of action if its your thing. While you are there, the miss beacon plumbing and oberto hydro shops are there. If the doors are open they will usually give ya a tour.
If you end up in Portland in November the Portland Train Club has tours of thier 2 story train layout..It is huge.
This weekend is a Train Exibition at Puyallup Fairgrounds. Huge layouts and a perfect way to wear out the little people. When mine was 2 I would get him the engines that dont work and the cars for a dollar at the swap meet. He would push those things around his track for hours. There are many real trains to ride around washington. We did the steam train at Mt Rainier historical train place.. You can google it.. We found that the monroe watching is only good early morning on sat.. very few run on sunday. There are more train rides in portland . These train brains had me driving all over everett chasing the steamers that BN runs from oregon and back.. They even had me bring the boat so we could follow them down the water front. I paid em back.. 4am, 30 degf, salmon fishing haha.
Good luck!