Notes on these missions: 7. The Great Train Robbery 23m48s. (starts 0:05) Not much to be said here. The 8th train comes out of the center gate at 23:40. The only thing I can do end the mission more quickly is to blow up that base and cram a whole bunch of units up against the door to blow up the train the instant it spawns.
8. Cutthroat 5m43s. (starts 6:38) Once you buy Mira's aid, her starting forces can immediately walk over and win the mission. The only goal of the first half of the mission is to get to 6000. I start making SCV's immediately until the Vultures arrive and I get supply capped (at that point it wouldn't be worth it to make more). The only unit I add to my starting forces of 5 Marines is a Diamondback. The starting Marines can take out some enemy Marines near the beginning, but they can't deal with the Firebat/Medic easily, and would take a long time to kill the CC anyway. One Diamondback can tank for the low-HP Marines while they finish off that base (some micro required here). On the Vulture side, a few mines deal with a scripted Marine attack without taking damage.