Question: I think there should be a specific term for someone getting a free ride without consent, but I keep thinking of the word "hijack" which clearly isn't correct.

Examples include jumping on a train without a ticket, or riding without consent on the back of a truck. Or perhaps I want to sit in on someone else's meeting and just observe.

Am I gatecrashing? Or going along for a free ride?

Equally, if I hop on the roof of a train to get somewhere for free, is there a term for that?

Answer: 
In the UK at least, someone who rides public transport without paying for a ticket is called a ['fare dodger'](https://en.wiktionary.org/wiki/fare_dodger)

> 
> Fare Dodger - **noun**
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> 
> 1 - A person who deliberately avoids payment for public transport
> 
> 
> www.wiktionary.org
> 
> 
> 

However this term does not apply to someone riding without consent on the back of a truck [or other vehicle].
When we used to grab onto the bumpers of unsuspecting cars for a tow while skateboarding, we called it ~~*hitching*~~ *skitching*.