I'm playing space exploration, where i'm running trains on different surfaces, and it seems like the train log interface doesn't even display that, nor recognize it's happening.

I've never played Space Exploration myself and had a use-case for this, but you are right that it would be a useful feature to add. Suggestions for how to do the filtering? Checkboxes for each surface, or a dropdown where you can either choose a specific surface or choose "All surfaces"?

surfaces can be a lot, so checkboxes doesn't sound that useful. also in my experience there isn't much reason to consider anything but all surfaces or a single one :)