With little experience try youtubing cakes. 3D? cakes for a is easy to cover up. Ice it all down use cookies, for wheels, candies or fruit even to fill the cars.
And if all fails grab a train at the toy store and put it on top of the cake. One year, i put a Thomas motorized train around cupcakes that looked like a mountain.
Happy Frosting.

Fran's Cakes & Candies in Fairfax City can probably help you out.

Michael's or AC Moore both sell the basics (Michael's at 7 corners and AC Moore is in Bailey's). I doubt you would get someone to give you tips or advice unless, like a PP mentioned, you take a decorating class. Fran's cake and candy supply in FFX is the closest baking supply store I have found in NOVA. Have never asked for advice as they don't give off that vibe but they do offer classes as well. What I have done in the past is a basic google search on what I'm trying to accomplish and then try and find someone who has given some sort of tips or at least a decent picture to use as a guideline. The upshot of decorating a cake for a 3 yo is they are impressed regardless so coat that puppy with frosting and use candies as your "embellishments". I've found that candy vs. trying to pipe details will save you tons of aggravation. Good luck!

PP here. Oops, just saw you said Falls Church. Check VA locations!

I think Michael's stores offer Wilton cake decorating classes. Check at the one nearest to you. I don't know of any in DC but there are a few in MD.

I want to make a 3D train cake for my son's 3rd birthday, but don't have much experience with icing etc.
Can you recommend a store where .i can buy the products and also get some advice?
Thanks
Eva