I The word "rationale" refers to a logical explanation rather than to a plain reason, and it can be quite long. (sense "1" below)
(SOED) 1. A reasoned exposition of principles; an explanation or statement of reasons. 2. The fundamental or underlying reason for or basis of a thing; a justification.
For instance, you say normally "The reason for my being late is that I couldn't wake up in time.", but you would never say "The rationale for my being late is that I couldn't wake up in time."; the logical consequence is just too plain for using this latter word. This is why there is a great number of books, articles and papers with titles mentioning the rationale of some entity (The Rationale of …).
However, the meaning of "rationale" tends towards that of "reason", as in sense "2".
(ref.) The rationale behind the introduction of these credits was primarily to reduce the dependency of the United States on oil and natural gas.
They cut off our water supply, but didn't give us their (reason/rationale).
The word "rationale" wouldn't be used in such a context; there is usually a simple, direct cause-and-effect event (work on water supply network, nonpayment, water shortage, …) that results in the supply being cut off.
II "Reasonable" implies qualities that are to be associated more with general wisdom than cold logic, which is the true province of what is usually labelled as rational.