As mentioned by others, as one gets older, the ability to sleep comfortably in coach gets less every year.
Just about every year for the past 9-10 years, due to WAS-BOS connections for the Cardinal, I've spent overnight in coach...business class, to be exact. Due to late arrival of #8 at CHI several years ago, I missed my connection and went coach on the Lake Shore Ltd. All the rest of the time my LD travel is in roomettes, mostly paid for with AGR points.
If I know I'm going to spend the night in coach, or even longer day trips on the NEC, I travel with a couple of inflatable pillows I found at Amazon. It takes two to 'stuff along the wall' to provide extra arm support as well as to prevent the pillow for your head sliding down along the wall. I also have a very compact blanket that packs into a pillow-shape from Amazon as well. If traveling in winter, my overcoat becomes my blanket. If traveling in summer, have a long sleeve shirt handy as sometimes the A/C is set to 'deep freeze'. I also pack my fuzzy lined moccasins and put them on when I want to be comfortable in a roomette or spending the night in coach.
Ideally, I have the entire seat to myself. Sleeping diagonally with my feet in front of the aisle seat works pretty good. In my own experience, 2-1 seating on some of the trains with business class provides the large armrest between seats on the '2' side, but there's less legroom in those seats than LD coach! My own preference is 2-2 seating in BC or coach.
Of course, the downside of overnight in coach is the possibility of getting 'Fat Albert' next to you, or 'Smokey Joe' or any other types of difficult/noisy/smelly seat mates. The few times I've had a seat mate, they were quiet and didn't smell bad.
I also learned the hard way that when selecting a seat (if given the opportunity), look where the lights above the aisle are. Position yourself as far to the rear of one as possible, perhaps just ahead of the next one. Even though (hopefully) dimmed, having the ceiling light directly in your eyes is a real problem. I've also witnessed other coach passengers use some tape and a piece of card-stock or thin cardboard to act as a light-block. I wish I had thought of that and had tape with me!
As one respondent mentioned, on Superliners, don't be across the aisle from the steps area. Foot traffic to the toilets downstairs and lights can be annoying. However, the first seat in front of the steps on the same side is probably the best spot to be (subject to overhead lights). However, check its reclining limits before taking that seat. The times I ride coach (mostly NEC), I position myself mid-car for the best ride, and if unavailable, then further away from the toilets (traffic, smells and door slamming). If you can sit anywhere on the train, be as far away from the lounge car as possible to reduce traffic past your seat. And, although I've never done it, there's nothing to stop you from moving seats (and cars) after the conductor that forced you into a specific seat and/or car is replaced several hours later by a new crew. Just remember to take your seat check with you. Of course, if you've vacated one of the two cars that normally get spotted at a short platform that's your destination, it'll be up to you to walk back to the right car to get off.
Lastly, you may want to consider using 'shorts'. Ride coach during the day and switch to a roomette after 9PM or so (or 6PM for free steak dinner!). Check the train schedule and determine where it is a bit before you likely want to sack out and book that segment until 9AM or so in roomette, then back to coach. Most agents won't know what you're talking about. When booking online, use the 'multi-city' option. I had to do that to make a one week prior to departure schedule change to take #422 LAX-CHI. I moved from LAX-ALP roomette to the family BR ALP-DAL and back to roomette DAL-CHI. I booked it online as 'multi-city' and managed to keep it all in the same through sleeper by specifically selecting tr 422 for each segment.