Other than the city effecting how fast the strats will rebuild, and it takes a lot of bomb tonnage, which then effects how fast objects rebuild on feilds, this moves into the realm of one guy with a bomber can shut down a country for x amount of time. You may have noticed Hitech does not allow that and why he changed the tonnage and down time for the HQ at the end of AH2 for being abused that way. Yes, it was his own oversight, but, our game was not so grifertastically focused years earlier when he set the tonnage and down time for the HQ. It may have cut into his bottom line, customers were that mad almost every night once that greifing act caught on.
A rail yard will never be as big as the city, and the city as it is reduced in capacity gradually effects the targeted country in the manner you wish, or sort of. Incorporating a rail yard into the city as part of the target package, then what does it effect? With my new terrain I will have to install a resupply path system. Fields get roads or river resupply, strats get rail resupply. Once you test setting it up it becomes obvious how Hitech wants these things to work for everyone.
Right now the city as an aggregate effects all strat equally in percentage as it is destroyed towards 0%. If all it takes is targeting one small area of the city to impact all of the strat rail resupply, once again you are back to one guy with a bomber dictating outcomes for a whole country. That did not happen in WW2 other than using a nuke. A box of bombers with 1000lb bombs is only a fraction of that projected force. Hitech is choosing to limit your ability to project force against a whole country. A box of bombers with 1000lb bombs is not equal to a nuke.
You are effectively asking for a country wide choke hold taken out from 30k with a single button press. In the past that is probably the core reason none of these repeated requests for targets like this were acknowledged. And the HQ debacle was a taste of how bad that kind of thing can be.
You could ask for each strat resupply rail to origin at a rail head strat kind of object. Right now you have to get up close and personal to destroy a train, it won't work from 30k in a bomber. That only effects the current resupply run during that 10 minute window while making you vulnerable to radar, interceptors, and the AI guns on the train. So bombing a rail head from 30k, would Hitech harden it so it would take a fully loaded box of B29? Or would he make it rebuild in time to launch the next train in it's 10 minute time schedule?
Based on the current functions and rules for the automated strat resupply, it gets sticky trying to shoe horn in this kind of strategic target for a 30k bomber and a single button press. With all of this species of wish, no one tries to fit it into the current model becasue they want a strategic choke hold result to replace the gradual degradation system currently in place. So it's generally they hope Hitech will pull something out of his yazoo. Try analyzing the current system so that he can fit a new object into it that works with the current rules.
Or outright wish for choke hold objects that you can bomb from 30k and impact a whole country with a strategic button press. Strategic bombing in WW2 took a large number of bombers, not one guy from 30k pressing a single button to shut down all rail function and resupply for germany. The current system reflects how little force a box of bombers can project against large complexes, versus how powerful they are against a town or an airfield.