Fieldwork is 2 blocks from the San Mateo Hillsdale CalTrain Station. They're opening a ROAM burger joint next to the Blue Bottle and Tin Pot ice cream store across the street. Will be a nice little one-stop-shop soon. I'm there a lot because it's close to my house and I'm a fan of the larger outdoor venues. I mean, I'll still hit Habit over Roam 100% of the time, but it's nice to see options.
Steve if you come up for Fieldwork, El Sinaloense in a good sit-down Mexican joint in San Mateo close to the Hayward Park CalTrain station (1 stop from the Hillsdale Station). If you're willing to go a bit further, you can train into downtown San Mateo (2 stops from Hillsdale) and hit La Cumbre which has good food but is a little expensive (e.g., you pay $0.50 for chips). El Nayarita used to be solid, but the service went downhill and the prices are too high to justify the rats running around outside and occasionally through the kitchen. Also there's been a couple violent-crime incidents in the 7-11 that shares the parking lot so we just avoid it (just FYI in case someone recommends their tacos).
I've heard Tacos Al Vapor is solid, but honestly we usually hit La Cumbre (the "home of the Mission-Style Burrito" as seen on Bourdain's show) because their grilled meats are really good. Probably the best grilled chicken burrito I've had in the Bay Area.
If you don't come up to San Mateo, El Grullense E&E (near Gourmet Haus Staudt) is great. Alternatively, GHS has great food.