One of my 'tricks' to get (and keep) a full seat to myself is seat location in the car, as well as which car to ride (if given a choice). I've ridden more than enough NEC trains as well as LD trains in coach and have made a 'limited mental study' of the human nature of passengers. As a result, I decided to forgo my 'center of the car' for a better ride for 2-3 rows from the rear end of the last car on the train (on the NEC, the BC car is last and the quiet car the next car forward).
Why the last car? Because at full-length platforms (on the NEC, mostly), most boarding passengers will be closer to the center of the platform. Very few congregate near the ends of the platform, hence, making the choice of last coach (quiet car or business class) automatically reduces the number of passengers looking for seats in that car. If only the coaches near the center of the train are on the platform, I walk through to the last car. Only a very small percentage of passengers will walk back (or forward, if coaches in the front of the train) to find a car that is sometimes nearly devoid of passengers. Oh...by the way...If you're forced to sit in a specific car(s) because of destination (such as 448/449, 27/28 and 421/422), if you know where the conductors are replaced by a new crew, grab your seat check and move to the last car as the passengers are boarding at that stop. A new conductor will assume that the previous conductor permitted you to sit in a different car than the rest of the passengers for that destination for a reason and not hassle you about being in the 'wrong car' (unless you ARE incorrectly seated in any of the 6 trains/blocks indicated above.
Why the last 2-3 rows of the last car? For starters, most passengers will board at the forward vestibule of a car (on the NEC). Experience shows that most passengers will choose the first open seat they come to that doesn't already have someone at the window seat. As more passengers board, they will migrate more towards the rear of the car and seeing that all those double seats already have someone in them. However, 4 out of 5 times they'll turn back and find a single seat in the middle of the car. And for those boarding passengers that got on one or more cars ahead that can't find a seat to themselves and keep walking back, the same 'rules' apply...they'll take the first open seat they can sit alone.
One more trick...have a small suitcase on the vacant seat next to you for easy access as well as to provide an armrest. If you really want to get tricky, pretend to be sleeping as well. Both of those tend to lead boarding passengers that either there's someone next to you (in the lounge car, perhaps) or they want to be polite and not disturb your sleep. But if someone wants to sit there, by all means be polite and move your stuff off that seat for them.
On commuter trains such as Metro North or Metra whose trains originate at stations such as GCT or Union, the rearmost cars - the ones closest to the gate, will always get the most passengers as they don't want to walk any further than necessary to get a seat. So, I do the opposite, sit in the lead car. At shorter platforms along the route, the conductors/engineers will frequently put the rear of the train at the platform and the first car or two beyond. They may also 'switch up' and board the lead cars at some stations, and rear cars at others. Experience on a particular route will show you which cars get the least amount of passengers. And don't forget that some commuter train conductors will tell you to be in the first 4-5 cars for your station as only those will be spotted at the platform. For Long Island and NJ Transit trains at Penn Station, I'll always head for the front car, as it's usually the furthest away from the steps most passengers use.
Of course, there will be times where I'm the one getting on mid-route and the last car is almost full. There's no fully vacant seats, so it's pick one with a skinny person in it. I'm 50-60 pounds overweight, and sitting next to a skinny person makes the journey much more comfortable. Remember, they, too, may be using the same tricks to discourage me from sitting next to them.
Almost forgot...if the toilets are at the rear of the car, take a seat at least 3-4 rows away from the toilets, as the traffic past your seat and 'blue chemical' smell can become unpleasant.
