Thanks for replying, RussianMatreshka!
I booked tickets to Arkhangelsk using rzd.ru - I entered Moscow to Arkhangelsk and the system showed two trains, the morning train to Arkhangelsk and the evening train to Severodvinsk, with Arkhangelsk as a stop along the route. We had to take the later train, and I assumed that it meant Arkhangelsk city centre, but when the ticket was paid for and confirmed, it showed the station as being Isakogorka instead.
I double checked on the Real Russia train bookings website and it showed the same train when I entered the route Moscow-Arkhangelsk.
Unfortunately we can't change our train, so we'll have to get into the centre from Isakogorka. I know that it is not too far from the Arkhangelsk city centre - so I assume that there is some way of getting into town quickly from there. I'm finding it hard to download bus maps and read the public transport schedule for the city - and I can't speak Russian well enough to pre-order a taxi!