Examples of rail assemblies for architectural coverings disclosed herein stabilize motor assemblies via end caps to reduce transfers of vibrational motor forces to the rails. An example rail for an architectural covering includes a housing, a motor to be disposed in the housing, and an end cap to be coupled to said housing. The example rail includes a track defined in an exterior face of the rail. A first portion of the end cap is to be received in the track. The end cap is capable of at least two degrees of freedom of motion relative to the housing prior to the first portion being received in the track. In the example rail, the track is to restrict the at least two degrees of freedom of motion of the end cap when the first portion of the end cap is received in the track.