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 [stowaway](http://www.collinsdictionary.com/dictionary/english/stowaway) is:

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> a person who hides aboard a vehicle, ship, or aircraft in order to gain free passage
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(Collins Online Dictionary)

From Alexander Rein's *The Blue Streak: A Hacker's Guide to Special Relativity:*

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> For example, if a railroad train is passing by a bystander who is very close to the tracks and at the same time a stowaway on top of a railroad car is running in the forward direction, the Velocity of the stowaway relative to the bystander is simply the sum of the two Velocities: the ground velocity of the train and the Velocity of the stowaway relative to the train.
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"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!