The most iconic part of a steam locomotive is the pilot, something I was reminded of in a recent thread; where I looked up the the "Ross-Meehan" pilot.

I've always wondered why it was that certain railroads used pilots as "signatures". Pennsy's "chicken coop" certainly identified their engines (at least until they started to use the solid steel ones with the drop coupler". Western Maryland's seemed to anticipate the occasional snow, and the D&H's vertical tube boilers, seemed spindly and inadequate; especially on Challengers.

Does anybody know how these designs emerged and why they were copied like a signature, even when cast pilots were available from Commonwealth and others?

Frisco for the most part stuck with either the boiler tube or flat strap pilot. However when they started doing some streamstyling and rebuilding engines in the '30s they concocted a couple of different designs. Frisco enthusiasts called the long nosed one a Chicken Coop. The flat one didn't contribute much to the 4-6-4s as they had Coffin FWH and the smokebox was longer, but with the longer version it looked nice. They used the longer version on a lot of unstreamstyled engines.
The 4300s were built with the same pilot casting that the C&NW used on a lot of their engines.

I think they are there to catch cows. Or equivalent.
(mechanical equivalent that is).

Ross Meehan was a foundry located in Chattanooga, Tenn. I don't believe they had their own design of pilot, they just merely cast what the customer requested. The NC&StL 576 in Nashville has a pilot cast by Ross Meehan.

My understanding, from several books, is that the Western Maryland wasn't as much concerned with snow as it was with rocks, falling onto the track from various cuts in the mountains. While those little 'plows' could move some snow, they were referred to as 'rock knockers' on the railroad.

Also, Vernon Smith, in his autobiography, made a comment on those long Santa Fe pilots. He said it made things interesting for the hostlers in the engine terminals, as they couldn't move power around by coupling plot to pilot without damaging the pilots.