The origin, usages, development and corruption of slang words and phrases has been one of my interests for many years.
The term "missed the bus" has been used to refer to failure for at least 80 years. The British Prime Minister at the beginning of WW2, Neville Chamberlain, seemed fond of the term, having used it in private letters in 1938, and in speeches in 1940. "Hitler has missed the bus" he opined, referring to the failure of the Nazis to exploit their military advantages in the late 1930s. Whether he was the original source, or whether he was using a term already current is hard to say, but offhand I know of no earlier recorded example of it being used.
After the German invasion and occupation of Norway, when German forces decisively trounced the British and French armies the phrase came back to haunt him, and in a May 1940 speech in the House of Commons he was taunted with it by Opposition MPs. When the Leader of the Opposition, Clement Attlee stood to make his reply to the Prime Minister's statement, he said something like, "The Prime Minister has talked about Hitler missing the bus. But he and his Ministers have missed all the busses since 1931. They missed all the peace busses, but caught the war bus." (Caveat: I may not have the exact words.)
So at this stage (7 May 1940) we have the term "catching the bus" as a euphemism for success, verified by Hansard, the British Parliamentary official record of proceedings in Parliament.
Naturally, in the normal manner of the development of vernacular language, the terms "missing/catching the bus" became regular slang usage for failure/success. At some point somebody has used it in the context of succeeding in an attempt at suicide, and, well, it's caught on here.