I think they are done "first come, first numbured"
LAL 420 was the first C420 on the roster, so it got the number.
same with 425 and 430.
Im not sure if 424 was the first of the Guilford units placed back in service or not, but if it was, the theory holds there too.
The current LAL 421, 422, 423 and 424 dont seem to follow any logical sequential pattern based on their predecessor or original road numbers,
so it makes sense that 424 was the first back in service..
although, if that is true, you then have an odd numbering sequence where 424 came first, followed by 421, 422 and 423..
but you cant logically go from 424 to 425, since 425 already existed.
so it makes sense to me that 424 would have been the first back in service, and 421, 422 and 423 got their numbers to fit the sequence, even though they were "backwards" after 424..
But for the most part, the theory works.
hold on...no it doesnt...because among the C430's, 431 and 432 were operating long before 430 was! although 430 was the first painted in LAL colors! although did 430 (a susquehanna engine) arrive on the property before 431 and 432 did?? (the M&E engines)..if so, then 430 can be said to be the first LAL C430, even though it wasnt returned to service for many years, it was still the first on the property, making it thew logical choice to get the 430 number..so maybe the theory does hold there..
(actually, the two susquehanna engines arrived together..maybe 430 got the number because it was deemed the most likely to return to service first of the pair..which it was...same with the Guilford engines..they arrived together, but maybe 424 was given the number because it was the first worked on and the first to return to service with LAL.)
As for the new WNYP 636, it probably got the "class number" simply because it is going to be the first of the Cartier units to operate with LAL..
the predecessor numbers dont, and dont need to, have anything do with the eventual LAL numbering scheme..
please note that all of the above is pure guesswork on my part!
just trying to work it out logically..
Scot