The Crossrail station doesn't aim to merge the Euston Road mainline terminals into one huge station. Rather it exploits the recent trend of subterranean stations having two entrances to be able to serve both from the same platforms. It is likely that some passengers may choose to use the Crossrail 2 infrastructure to transfer on foot between the two terminals, however, especially in the rain, and also plausible that the design may be tweaked to make this more attractive, perhaps incorporating a separate connecting passageway between the entrances parallel to the platform tunnels, maybe even with some form of moving walkway technology for part of the distance, but Euston and St Pancras will nonetheless remain separately identified and managed terminals no matter how easy it becomes to walk quickly between them.
Picking up on the Euston Road-linked 'main line' stations, and sorry if it's all been said before (in which case I'm sure I will be corrected, hopefully gently 'advised') ...
My surmise is that more people will 'interchange' when maps show them that way and that represents a physical connection without having to 'go outside' to leave one station to find another, eg Euston to Euston Square, unless and until the traveller develops confidence from familiarity. I would hope that 'the authorities' have data. As a kid I wondered a lot about eg West Hampstead (yes I know ...) without thinking WHY would anyone (or many) change there?
I imagine (hope) that there is also a lot of data re e.g. Northern Line overcrowdings, origin-and-destination studies, development 'potentials' and areas less than well-served at present, human behaviour studies, to name just a few 'considerations'.
Feeling for Michele Dix and her advisors seeking to balance these often conflicting desires and demands, and make progress 'at this difficult time'.
