Originally Posted by
psychokilla
Is that a hard and fast rule, Oxon? Left to right, you'll end up in Euston Square, top to bottom, you'll end up in Euston? I'm not sure what else I'll be doing that day yet, so just wondering it's set this way all the time
Euston Square is actually a different Underground station from Euston Underground Station. Euston Square is served by the Hammersmith & City, Metropolitan, and Circle Lines. To get from Euston Square Underground station to Euston Station (mainline or Underground) is a short walk at street level: follow the signs from Euston Square Station.
Euston Underground Station, on the other hand, is served by the Victoria and Northern Lines. It is underneath Euston mainline station and the two are connected by escalators and stairs: no need to go out into the street.
Given that you don't know any of the stations involved (Euston Square, Euston Underground, or Euston mainline), I'd allow a good 20 minutes to make the transfer. It won't be necessary, but better to be a bit early than to have to rush or miss the train.