D&RGW baggage cars and roof walks

I've seen mention that the roof walks were for baggage cars which were also used on mixed trains. That would suggest they were a later feature which came about when there was no longer justification for separate passenger/freight on a particular line.

But I'm also aware that end doors in baggage cars had some chronology (although I'm not sure whether they were deleted or added as time progressed). Did roof walks also get added when end doors were deleted (or removed when end doors were added)?

Any info on the timeframes of any of these changes would also be appreciated.

Cheers,
Jeff.