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:

> 
> a person who hides aboard a vehicle, ship, or aircraft in order to gain free passage
> 
> 
> 

(Collins Online Dictionary)

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

> 
> 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.
> 
> 
>
This is getting a bit away from your question,
but, if you observe a meeting or other activity without the consent —
and, especially, without the *knowledge* — of the participants,
you are ***snooping*** or ***spying***.  If you are primarily listening
without the consent and the knowledge of the speakers,
that's ***eavesdropping***.

On the other hand, if you overtly attend a meeting in order to obtain refreshments to which you aren't entitled, that's [***mooching***](http://dictionary.reference.com/browse/mooch),
or simply ***stealing*** (or ***theft***).