There is an unwritten code of polite behaviour when travelling on the train. It is a complicated code, the nuances of which are generally only understood by the most seasoned travelller. If you do not wish to stand out as an obnoxious interloper, the following may help:
Train Etiquette for the Uninitiated
Music- nobody wants to hear yours, no matter how kicking your tracks are. Take the earbuds out of your ears. If you can still hear the music, or if anyone is looking at you, it's too loud.
Telephones- for the love of everything good and pure, shut of your keypad sounds. While you're texting, every beep boop boop puts you one second closer to an elbow in the face from anyone within range.
Just because your voice is transmitted to space and back doesn't mean you have to shout into your phone. Try to speak quietly if you must take a call. Better yet, don't take it at all. It will inevitably go something like this: "Arright mate? Yeah, like, where we meeting like? Gazza was so pissed last night. Hello? Hello?" Ring ring. "Yeah mate, on the train. So, like, I was saying...hello? Hello? Damn". Just give up and send a text. With the sound off.
Personal space- you get one armrest, and it's on the outer side. The one in the middle is not for arms. It's a divider between you and the other person.
Hygeine- showers, deodorant, and toothpaste are daily essentials. Wash your coat and jumpers, preferrably more than annually. No perfume. You may think it smells nice, but it only smells like rose scented cigarette ash and dog to everyone else.
Table seats- these seats are meant for four. Therefore, unless otherwise mutually agreed, you get one quarter of the table and floor space. With eye contact, you may initiate the complicated game of footsie with strangers that will allow you to negotiate an interlocking foot arrangement that will allow you to stretch your legs.
Personal items- Translation: "May I put your bag on the rack for you?" means: "Your stuff is in everyone's way, you selfish pain in the arse". If your bag did not pay for its own seat, move it. Put it on a rack, not on the floor under your seat. That area is not large enough to accomodate legs, let alone legs and your handbag/luggage/shopping/duffel bag full of body parts.
Seating- regular travellers have their "spot". Please try to respect this when choosing a seat. However, if it makes logistical sense for you to have an aisle because you will be getting up soon (for a station, toilet break, etc) you may wish to mention this so as not to be awkward.