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: 
"Watch out for **Hop-Ons**!"

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> Hop-ons are people who jump on to the back of the stair car for a free ride. They are a constant annoyance for the Bluth family members who must drive the stair car. [(source)](http://arresteddevelopment.wikia.com/wiki/Hop-ons)
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![Stair Car Hop On](https://i.imgur.com/BUawS5f.png)

If your vehicle was towing a small cabin, you would of course need to watch out for **Live-Ins**.

This is a running joke from the TV series *Arrested Development*, but "hop ons" would be an easily understood term for the situation. You even used it in the question!
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