Steelbeast wiki helps on this matter. At least if this is correct.
http://www.steelbeasts.com/sbwiki/index.php?title=Infantry_UnitsRoute tactic behavior
Tactic Behavior
Engage
infantry will advance to contact and once in contact will stop and engage if able to do so
SLOW routes will cause the infantry to advance by crawling
FAST routes will cause the infantry to advance by jogging, until exhausted
TOP SPEED routes will cause infantry to advance by sprinting, until exhausted
Assault
infantry will advance to contact and once in contact will continue advancing by crawling while under fire
SLOW routes will cause the infantry to advance by crawling (before under fire)
FAST routes will cause the infantry to advance by jogging, until exhausted or under fire
TOP SPEED routes will cause infantry to advance by sprinting, until exhausted or under fire
March
infantry will advance to contact and once in contact will stop and engage if able to do so
regardless of route speed setting, infantry will advance by marching
Retreat
infantry will attempt to break contact with the enemy and will use smoke grenades if available
SLOW routes will cause the infantry to retreat by crawling
FAST routes will cause the infantry to retreat by jogging
TOP SPEED routes will cause infantry to retreat by sprinting
Scout
infantry will advance to contact, then once established will observe the enemy
once under fire, infantry will automatically retreat back by crawling in the direction it advanced from
SLOW routes will cause the infantry to retreat by crawling
FAST routes will cause the infantry to retreat by jogging
TOP SPEED routes will cause infantry to retreat by sprinting
Breach
behaves identical to engage tactic
Note that infantry will stop and occasionally rest if they are on FAST or TOP SPEED routes, and they do this in order to maintain that higher rate of movement for as long as possible. Over time though, depending on the circumstances, they will likely become exhausted and will be reduced to crawling (for tactical considerations) until they are rested.