For anti-theft most trains have a full surveillance system coupled with an alarm similar to the ones in houses, which will dial the authorities when triggered.
For conflict management there is always a gunman aboard. Sometimes it is the conductors themselves, sometimes it is undercover police, sometimes it is hired security guards, and sometimes it is a marshal. How they go about it depends on what the train is doing. For example, freight trains usually have hired security or armed conductors because the marshals and police want to focus on securing people instead of cargo.
On which train and in which country?
In some countries trains are escorted by armed guards, in others that's not neccessary and school children ride on the train on their own without the need of protection.
There are conductors on the train, and they keep an eye on most people and report anything suspicious. ON New Years Eve and on days when they expect to have allot of extra riders, there are also "plain clothes" security people, some of whom are off duty policemen.
If you are in the US and a minor–be aware that train personnel are on the lookout for runaways. If you are traveling by yourself, you need to follow the rules for minors, and a parent or guardian must drop you off at the train.
I can only talk about Britain. Most of our trains have a guard or conductor or train manager on board. Most have cameras. All have passenger alarms which can be used in case of emergency. Apart from fare evasion, crime on our trains is rare
In Atlanta, we have police officers on our trains. They're uniforms employed by the transit company. They also claim to have plain clothes, but that's probably a bluff.
What kind of security do es the train have I am asking abut a minor?
Depends on what train you are talking about.
There are just officers and cameras.