I'm assuming this has resurfaced in light of the announcement that Amtrak is launching nonstop DC-NY service (https://www.usatoday.com/story/travel/news/2019/07/25/amtrak...)

The new service will run one round trip daily (southbound in the morning, northbound in the evening) and take 2 hours, 35 minutes - which is what the OP says was running in 1969.