The standardsisation goes back to the 1st generation DMUs. They were built by many different manufacturers yet had basically the same controls, same engines (a choice of 2 main ones), same gearboxes (with a few variations), same vacume brake, and in most cases the same multiple control system and therefore compatability.
The 15X series all use the same Cummins NTA855 engines, with the exception of some 158s which have Perkins engines. They all have Voith T211 transmissions, which are also found on the Pacers, 16X series, Turbostars (although not the 172s) and 175s. The basic control layout is the same on all of them. They are all compatable for multiple working, and 15X series can work with 14X series (Pacers) and 170s.