I had been wondering about that too, but hadn’t measured them. Because of your question, though, I just held a ruler to them. There is 41.5 mm between the inner faces of the rails, meaning they scale out to 1.45 m IRL. That’s under 2 cm out from standard gauge of 1.435 m, which is 41 mm in 1:35. So, the short answer is: any railway kit made to scale standard gauge should fit, but Russian (or Spanish) ones will not.