Check out the Boarding Actions rules on page 67.
Summary
Grapple Check - Infantry gives up it's attack and rolls 2d6. On 6+ the grapple is successful. (Note: this is for Large and Very Large units like dropships so making a physical attack may be more appropriate for smaller units.)
On the same round as the Grapple Check, boarding combat begins. All infantry that successfully grappled and opposing crew/infantry make standard attacks at short range. (Note: in the case of combat vehicles, you can just assume 1 point of damage will kill the crew.)
Example
So, let's say the scenario is a train heist. The attacker has a maxim with 3 infantry and ends its movement in base contact with the train car. Each infantry makes a grapple check. 2 succeed and then make boarding attacks. However, the train car has 2 platoons of infantry and the train crew (no combat stats). They make their attacks and no one likely dies the first round.
Second round, the train moves, the 3rd attacking infantry mounts the transport, and the maxim gets in base contact with the train again. Since the infantry mounted, it can't dismount this turn. The boarding infantry and defenders fight again killing 1 platoon each. Both sides are down to 1 infantry platoon.
Round 3, the train moves, the maxim moves into base contact and the 3rd platoon tries to grapple and succeeds. Now it's 2 attackers vs. 1 platoon. The two attackers kill the defending platoon.
Round 4, the attackers kill the crew of the train and take control.