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: 
A free ride is a ***[hitch](http://www.collinsdictionary.com/dictionary/english/hitch):***

> 
> * (informal) a ride obtained by hitchhiking.
> 
> 
> 

(Collins Dictionary)
How about "tagging along", meaning "to go ​somewhere with a ​person or ​group, usually when they have not ​asked you to go with them.

<http://dictionary.cambridge.org/dictionary/english/tag-along>