Answer:
Please see below as the answer is self-explanatory.
Explanation:
In order to be able to move in a straight line at constant speed forever, as stated by Newton's first law, the object can't be subject to any external net force that can change its momentum.
As it is assumed that the spaceship has negligible interactions with another objects, it will continue moving in a straight line at a constant speed, forever, so it's a good fit to explain Newton's first law.
Even though a spaceship actually can experiment a gravitational force from any mass close enough to it, as stated by Newton's Universal Law of Gravitation, in order to simplify things, in this case, we usually neglect any of them.
This can't be done with a car, a train, or any earthling object, which due to are subject to the gravitational attractive force from Earth, it is not possible for them to move along a straight line at a constant speed forever, as stated by Newton's first law, so a train or a car definitely aren't a good fit in order to explain it.
Moreover, a car, or train, interacts with other objects (the air, the road surface, or the rails, for instance) which means that sooner or later, it will come to an stop, so, for this reason, is not a good fit for that purpose.
