Postal History, Station Codes

I have the below shown letter in my collection which is addressed as follows:

From KHR station to be posted at BMG station

What does KHR station and BMG station stands for? I assume they stand for a Railway Post office, but for which one?

Another letter indicating KHR Station

In addition I have heard about the a KBH station. Is someone having cross reference list of Station Codes versus Railway station names? What I have in mind is to create a cross reference list like: