FlyingForFun: Yes, there most definately WAS a career path to the flight engineer's position, in the UK at anyrate. You joined one of the airlines as an engineering apprentice, gained your licences, got some experience, then if you fancied it, you applied to become a flight engineer. This meant passing a medical, selection tests, etc. It worked primarily in BOAC, but also in British Eagle, BCAL, Monarch, etc. The alternative route was via the RAF and a considerable number of flight engineers were recruited this way by the UK airlines, especially in the 1960s/early 70s.
The term flight engineer in the USA is generally a misnomer for a pilot operating the systems panel on a 727 or somesuch, waiting for the seniority number to come up and allow promotion to the right hand seat. In the UK, such a person on the panel of a BEA Trident, for example, would be referred to as P3 and in this instance, would be qualified and interchangeable with P2. (Is that clear as mud now?)
With the demise of the 747 Classic, DC-10, 727 and TriStar the job of the specialist flight engineer is rapidly disappearing. With one or two disappointing exceptions, they are a wonderful bunch, but I suppose the same applies to pilots!