I just bought the kit yesterday and haven’t assembled it but on the base, the plastic rail clips are exactly 3 inches apart. That is measured from left rail to left rail.
I like Walthers kits but they have a lot of kits where it is important to know the track centers and that is information that could easily be included on the box and in their catalog. I don’t know why they leave that out.
I have the Diesel fueling facility on my layout. I actually kitbashed mine by taking a razor saw and removing one of the bays to make it a single:
(Left click to make image larger)
This particular pic is an unfinished kit. I’ve since added the water and fueling hoses, as well as painting and some weathering. Now all that’s left is putting down some appropriate oil and fuel stains.
The 3" CTC width just didn’t fit my side-by-side steam/diesel servicing terminal so that’s why I trucated it. In the pic, I kept the discarded cement platform and added it to it’s length. I’m not sure if I’ll keep that or not but it doesn’t look bad.