I too have wondered about pronunciation, however, we are talking about what in essence is just a number. I have heard people use for example both "four-two-one-o-three" and "four-twenty one-o-three" to describe 42103. The drivers I have heard use the latter typically.
Association is a strong influence on people whether they realise it or not. It is often seen when people move or spend time in another country they tend to copy the speech patterns (phraseology and accents) of the locals; is it right? Whose to say what is the correct way, but most people will, usually unconsciously, seek the affirmation of those to whom the speech pattern is native.
Many years ago when visiting Brisbane I decided to take a ride to Beenleigh. Not having ever heard anyone say the name I asked for a, "Return to Beenly". The Booking Clerk immediately corrected my pronunciation saying, "Been-lee". I had instantly identified myself as being an outsider, a non-local.
I am currently having a deck built at my home, using Merbau decking boards. I was talking to a friend in NSW about it and he questioned my pronunciation. I explained that Victorians say Merbow, NSWelshmen say Merboo; which is correct?
The NSWGR/PTC/State Rail/Whatever, documented a great many things in their voluminous procedures but pronunciation was not one of them.
Say it as you like, but if you want to be seen as 'belonging', say, "four twenty one oh three".