The only places in WWII where bombing was really effective against railroads was in Italy and France where the damage was done by fighter bombers, Typhoons and P47s mostly. The reason it worked then is because the attacks could be repeated day after day so about the time the line was fixed here came the fighter bombers again.
Another problem with hitting rail lines with level bombers in WWII was the the targeters tended to go after bridges and marshaling yards where repair supplies where stored nearby, instead of out in the middle of nowhere where the Germans would have to spend a day or two transporting repair crews and supplies before they could even go to work on the damage. When you have as little chance as we did then of hitting a rail line I guess they had to hope to make up for the inaccuracy with collateral damage.
The best examples of war against railroads is the US civil war. The Union never caught N. B. Forrest and the Confederacy never stopped W. T. Sherman’s Bummers but both sides got so good at repairing the damage that by the end of the civil war the strategic benefits railroad wrecking were much reduced.
I suspect the ever efficient Germans in WWII would have developed the skill pretty quickly - we might have seen prisoners from the camps hauled out to make repairs, then sent back to be executed or killed on the spot, the deaths blamed on the allies.
The cruising speed of a fully-loaded B17 was far less than 300 mph, but I take your point.