It's not a separate building. Just using the empty space in the concourse area. Also, the first floor is rarely used by travelers, actually a good place to setup the museum as people can just visit it while waiting for their trains.
The connecting line is now a distant dream as there is not enough space around SBC ( without demolitions) to construct this.
Also, given the extreme (really extreme!) amount of traffic around Majestic, it'll be disastrous if BMTC buses start...
more... stopping in the railway station. More so because BMTC buses always hang around the stops until the bus is filled. Even in KBS many times, buses take more than 45 min to start even after half the bus is full. But then KBS is as close as possible to the station and also the underpass connection is good, so probably no need of running buses right upto the entrance to just to save a 100m walk. Also, with all the traffic at KBS entrance, it'll take less time to walk to SBC from the bus stand, than wait for the bus to reach there. :P