I worked in the area when the North stand was being built, the club didn't own all the land in those days and they had to build over United Road without it being closed because it was the only access rout to some industrial estates, I had to drive down there most days and it could take anything between2 and 45 mins, it was a huge logistical problem
Looking at the aerial view of OT, the North stand would need to be demolished and re-built with a smaller footprint to allow any movement North, as for building over the railway, that would be a logistical nightmare and would cost a staggering amount, there's no way the railway would be closed down for any significant period of time, the land behind the railway is quite steep, it would need to be levelled out, quite a challenge with trains passing by regularly