Nice explanation. I was driving a locomotive built in 1873/4 last Saturday, it was in service until 1962 because it (and 2 others) fitted a particular job and it wasn't until then that a replacement was found. However it was changed over the years - I know roughly what it looked like when built, I've seen a picture of how it looked about 1910 and I know how it looks now - I don't think much is left of the original. Certainly since 1910 it has lost the feedpump, had a different boiler fitted and had the front buffer beam changed from wood to metal.
In other words it has evolved.....
Gavin
Linslade
Bedfordshire
UK
Garratt Locomotives
[
www.beyergarrattlocos.co.uk]