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)

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

However this term does not apply to someone riding without consent on the back of a truck [or other vehicle].
[Riding the rails](http://www.livinghistoryfarm.org/farminginthe30s/water_07.html) was the term, especially during the Great Depression, for "hopping a freight" to obtain free (though dangerous) transport across country.

There were variations. "Trainhopping" was another general term, and [riding the rods](https://en.wikipedia.org/wiki/Freighthopping#Riding_the_rods) was the term for the even more dangerous practice of riding on the truss and brake rods under the car (vs riding in an unlocked boxcar or riding atop a car).

[Fare dodging](http://www.collinsdictionary.com/dictionary/english/fare-dodging) is the somewhat more modern term for dodging the conductor or ticket taker on a train or bus.