Some Dell Rail Kits are interchangeable between server models and some are not. The 1U Servers usually share a common rail kit more than the larger servers. Additionally these are the entry level 1U Servers. Dell probably does this to reduce the cost as much as possible for this Server Line of products. Typically the higher end and larger Servers like the PE1950 and PE2950 have a Rail Kit that is very specific.
This was not always the case. The older generation servers like the PE2450 & PE2550 would share rail kits across generations. One feature that Dell has not changed is the attachment method of their rails to the rack. The Rapid Rail Hook is a feature that Dell has patented and no other manufacturer uses.