I would propose an alternative based more on technology levels:
Early Era - 1827 to 1860 : the formative years
Ante-Bellum - 1860 to 1875/1880 : standardization of gauges, train orders formative.
Turn of the Century - 1880 to 1910 : Transition to air brakes and knuckle couplers, cars jump to 36 and then 40 ft.
The Great War - 1910 to 1930 : Huge changes in car and locomotive designs, standard lettering, standard grab irons, steel underframes dominant, locomotives get trailing trucks, USRA operation, "modern rules", standardized waybills.
Depression era - 1930-1945 : Contraction and WW2
Transition Era - 1945 to 1960 Steam on its way out, dieselization, consolidation of rail systems.
Rust/Extinction Era - 1960 to 1980 : Class 1 railroads begin to die off, contraction, deferred maintenance, bankruptcies, larger mergers start.
Post Staggers era - 1980 to 2000 : Mega mergers start, railroads rebound financially, short lines proliferate, "traditional" rules replaced.
2nd Millenia era - 2000+ : Mega mergers stabilize, electronic technology proliferates, EOT, DPU, PTC, computerization widespread.