Maybe rebuild times could be inversly proportional to the size of a country's territory? This would make a steam rolling country more vulnerable, and wouldn't necessarily require more than one set of strats, as HT seems to prefer.
I believe Moot has hit the nail. This is one of the most important issues in a strategic system. Suply lines. In RL, this hampered all armies. German panzer units advanced so fast that they had to stop waiting for supplies (fuel and ammo). Russians had to wait weeks to prepare an assault. The germans had to lay thousands of miles of railroad tracks all over Russia. The rail gauge was different...
The longer are supply lines, the slower is ressuply. The shorter supply lines are, quicker the ressuply. Remember Stalingrad? The T34's left the factory unpainted and started firing less than a mile away. That's a quick ressuply line! Remember the Ardennes? Tigers abandoned after running out of fuel?
A steamroller wil occur everytime you manage to concentrate a massive force,well supplied, against a target weakened by strategical operations. Now, we have localized steamrollers, in sequence. You gather a enormous attack force, destroy everything, subdue defenders and drop troops. End of game, rinse and repeat. No penalty for grabbing land faster than you can build a railroad.
Gaining terrain? Ok, pay the price: longer supply lines, slower rebuilds.
Cheers