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: 
How about ***piggybacking***?

> 
> link to or take advantage of (an existing system or body of work). *"they have piggybacked their own networks on to the system"*
> 
> 
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OED
Specifically applying to subways (and possibly other modes of public transportation) is *turnstile jumping*.

This refers to the act of avoiding paying at a turnstile, which is a gate which opens when you deposit your fare, by jumping over it. I couldn't find a definition but it's widely used in [US newspapers](http://www.nydailynews.com/new-york/subway-turnstile-jumpers-saving-loads-money-caught-study-shows-article-1.162403)