I worked on three different RR electrification studies or proposed "rules" from the late 80's through the early 2000's. And, all of the cost benefit and impact assessments were based on a full 23 foot clearance requirement for modern double stack and auto-rack clearances. The RR's I worked with were adamant on that minimum clearance for overhead electrification.

This clearance and the necessary clearance (at least 1 additional foot) from the overhead structures is problematic for some older bridges, and added greatly to potential cost estimates, as in some cases, there might be very significant bridge replacement costs due to potential clearance issues.

As Kermit likes to say, it ain't easy being green........