Sorry Don,

But that bit about SRB's is wrong, you do not need to go through a tunnel to get from point A to the launch site. The problem has to do with weight on track not width of load. The track weight per yard was not over 100 lbs per yard for most of the main line tack in the U.S. when the Shuttle was designed. Today there is alot of 133lbs per yard even some as heavy as 156 lbs per yard. The problem is how to distribute the load so as to not kink the rail or have the track and ties sink out of sight into the balast. Also there were weight limitations in certain bridges that could have excepted the wider loads.