Is all Code 100 track the same? Is Atlas, Peco, etc. basically the same?
Between Peco, Atlas, and Model Power code 100 there are some slight differences, but usually not enough to be concerned with. Peco track, for example, is slightly narrower on the bottom, which makes a Peco rail joiner an extremely tight fit onto Atlas rail, and an Atlas rail joiner fits loosely onto Peco rail. Model Power flex track is stiffer than Atlas, and Peco is more flexible than Atlas. Both Peco and Model Power crossties are narrower than Atlas, which seem to me to be oversized. Once the 3 brands are down and ballasted, though, it’s very difficult to tell which is which. The actual height of the rail is the same between them.
But remember, the ties are different so you can not just have one peco and one Atlas without checking that they match or being forced to shim either.
Magnus
Not really–I combine Peco Code 100 switches with Atlas Code 100 track (flex and snap track) all the time and never have to shim–they match up just fine.