Minimum axle counts are not a stupid requirement. Shunting circuits efficiency vary by electric/electronics used along each corridor. If Amtrak or the States we’re willing to fund all the additional costs modernizing all the signals, minimum axle counts could be reduced. But they aren’t, and the freight railroads can live with the higher axle counts or reduced speeds of their trains.
DCTA installed a fairly modern signals along 21 miles railroad corridor, which works fine with 8 axles (2 RDCs) and 12 axles (2 GTWs) on a train, but had difficulties and further expenses getting the minimum axle count down to 6. Increase maintenance chores in scrubbing and grinding the rails are needed for proper shunting with 6 axle trains, which is far more more likely to be done on a rail corridor 21 miles long than on a rail corridor 210 miles long.
Don’t expect freight railroads to maintain their rails for passenger trains than what they need for freight trains free of charge.
Amtrak has chosen to add cars to their consists rather than fund upgrades to the signal systems. It was their choice.