Working on many projects the past few weeks I have made a lot of use of this handy tool, the NMRA OO/On3 standards gauge, which was introduced in 1981. [Update: It seems to be off the market now, but is totally worth tracking down if you are working to run American OO models.]

There is just no way to get your layout running well without this gauge. In general many OO wheel sets are not well in gauge and it will pay off to check them. In the photo I have included an example, a Nason tender truck. This particular one was very free rolling until I tried to correct the too narrow gauge of the wheel sets. When I had them out to gauge the truck would not roll! The solution involves adding a washer between the bolster mounting and a side frame. Problems of this type are common on classic OO models.