Here's a bit of a crack at a technical-legislative PoD from 1965, I don't know enough about US politics to make social shifts, nor would I want to. The idea is to have enough fast trains in service, with their double the average patronage, to make an impact.
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1965 High Speed Ground Transportation Act passed, includes appropriations until 1975, initial appropriation is $135 million. (OTL $90m) Included are several legislative changes to remove legal impediments to HSGT, most noticeably removal of the 79mph speed limit for passenger trains without in-cab signaling, and special consideration for trains purchased with HSTG Act appropriations.
1966 FRA publishes new speed limits for passenger trains on each of its track classes in light of the new HSGT Act legislation. On the most common class 4 track passenger trains are permitted to travel at up to 90mph and class 5 track in the West/Southwest at up to 100mph.
1967 Rail companies increase the speed of their passenger services to the maximum allowable by track condition and locomotive power. On common class 4 track average journey speeds increase from 45mph to 55mph. In line with the industry adage that every 1mph increase in average speed results in a 1% increase in revenue, passenger services see an increase in ridership of 10%. In response to this modest increase in revenue solely derived from regulatory change rail companies begin lobbying the FRA and various levels of government to get tracks reclassified at a higher level. This is somewhat successful on a small scale, certainly worth the minimal efforts expended by the Railroad companies.
1968 the first of 12 UAC TurboTrains enters US service for a cross country promotional trip after posting a 170mph speed record in December 1967. These trains were purchased via an innovative public-private partnership between the Dept of Transportation and the operating railroad companies. These trains are gas-turbine powered, lightweight, articulated construction and steerable, pendulous suspension. As a result they are able to use existing tracks and with high acceleration from their light weight and gas-turbine power and high cornering speed from the pendulous tilt and suspension steering able to maintain high average speeds despite a somewhat modest operating top speed of 105mph. The DOT intention is to bring TurboTrains into revenue service between Los Angeles-San Francisco, New York-Boston and routes radiating out from Chicago.
1969 the Electric Multiple Unit Metroliner train enters revenue service on the electrified line between New York and Washington. Metroliners are able to cruise at 125mph over 60% of this route and non-stop services are able to average some 87mph over this distance. Plans are being mooted to electrify the New York-Boston track and run Metroliners on this route at some 12mph faster than the current Turbotrain. Turbotrain pairs enter revenue service Chicago-Detroit and LA-SF, in line with the 1pmh+1% adage these trains see a 30% increase in ridership levels over conventional trains and 40% over 1966 levels when trains were hamstrung by the 79mph rule.
1970 DOT orders more passenger cars to increase the length of each train from four to six cars, increasing capacity from 200 to 300 passengers.