How was it before Amtrak took over?
My main experience was with what is now called NEC South, between New York and Washington, and mostly centered in Philadelphia, during the period 1966 - 1971. There was a frequent service, but, boy some of the rolling stock seemed like it should have been in a museum. On the other hand, some of the newer rolling stock was pretty nice. When you think about it, some of the older cars weren't much older than the Amfleet-1s are now, but they weren't air conditioned, they didn't have reclining seats, and the ride was noisy as anything. On the other hand, they had a whole lot of nice streamliner long-distance coaches from all the trains they were discontinuing, so it was always an adventure about what kind of car you rode in. They also had parlor cars, which I never rode, as the fare was over twice that of coach, and dining cars on some trains. The "cafe" service was what they called a "snack bar coach" which was basically a food cart at one end of a coach car serving overpriced pre-made sandwiches, snacks and drinks (35 cents for a can of coke you could get for 15 cents in any vending machine). You didn't want to sit in the snack bar car, as there was always a line in the aisle. Despite all our bad-mouthing the Penn Central, the cars were clean and in good repair.
After 1969, Penn Central also operated Metroliners along the route, which were more or less Amfleet EMU's, and the ride experience was similar to riding an Amfleet Northeast Regional today, except that I think the top speed was only 110 mph. Actually, the Amfleet design was derived from that of the Metroliner EMUs. Again, I didn't ride Metroliners too often because they were an extra fare train, sort of like what the Acelas are today.
The service itself was pretty frequent (hourly between New York and Philly and every 2 hours or so between New York and Washington), and most of the time, it was reliable, but when my Mom went down to Baltimore to visit family, she'd sometimes come back with tales of a train grinding to a halt somewhere in Maryland and just sitting for an extended period of time. I was able to do my joy rides to Trenton and Wilmington, and sometimes New York and never really had to worry about being late for dinner. As someone mentioned previously, coaches were unreserved, and you always risked having to stand most of the trip during busy periods.
For about four years after Amtrak took over, things didn't change too much, as the trains were still being run by Penn Central, although sometimes you'd see coaches from other railroads running in the NEC. The real change started after they started getting deliveries of the Amfleets in 1975, and especially after they did a whole lot of track work on the NEC in the early 1980s which allowed for 125 mph running, greatly decreased travel times, and more frequent (hourly) service between Washington and New York.
The only other pre-Amtrak railroad I rode was the Burlington's Denver Zephyr. That was part of a Scout trip to Philmont, so we were a charter group riding in older cars with our own dining car serving downgraded meals, but I was still impressed with the smooth ride, and, of course, enjoyed riding in the dome car.