These can't be missed so I won't spoil this (even though there's not much of a story).

Desert Railstar is the first five levels, Badland Railstar is the next four, and Cliffside Railstar is the last three.

To get three stars in a level, you have to have a time better than the three star time listed for the level. To get this achivement, though, you'll need some of the engine achivements. At the default speed, it's impossible to get the three start times on most levels. In the second level, "The Heights," you get buy the Boost Mk. 1. However, even this speed increase makes it difficult to get three stars on every level. It's not until the sixth level, "Trackswitch Boondocks," where you can buy the Boost Mk. II. That will allow you to get the three star times on most every level. Don't make the mistake I did and keep retrying levels without this second upgrade (unless you're just trying to get cash). And, if you have any troubles, just wait to try these achievements until you can purchase the Boost Mk. III on level 9, "Diesel Run." That will make the three star achievements trivial.

The golden envelops are barely hidden throughout the levels. I did not mark down their exact location but they should be fairly easy to spot as they are large and are radiating out light. Keep in mind that some weapons may not be able to reach them so use your trusty cannon to get the long distance ones.

You can unlock up to five contracts for every level. Some are unlocked just by reaching the level but others require certain upgrades. I completed Multitasker on the first level, "Cactus Downs," when I had both TNT and the Tesla Cannon. I could let my train stop at obstructions and fire off TNT at all the cactus, then continue on shooting at looters. Keep in mind that some of these contracts will actually be more difficult with upgraded freight cars because they can steal your kills.

However, this is (as of this writing) a really easy achievement with some perserverance because even though it says you have to complete five contracts at the same time, if you choose to retry instead of continue at the end of a level, you can retry with all progress for each contract saved. For example, if on the first level you only blow up 30 of 40 cactii you can hit retry at the end of the level and you will start the level again with 30 of 40 already counted. In this way, you can keep retrying until you get all five contracts completed.

Again, pretty self-explanatory. I will note the maximum train length is six (caboose, four freight, and the engine). Also, for Top of the Line, you can't do the last upgrades until you beat the last level with at least one gold star.

Hatches Battened should come pretty easily after you get your first freight car. If not, simply upgrade engineers until you can get through a level without it blowing up.

For Hotshot, I found it easiest to do this achievement when I had the alternate fire TNT weapon unlocked. It throws out five sticks of TNT instead of the single big one, make it easy to hit multiple foes. This achievement may be harder with upgraded freight cars because the enemies may not last long enough to get close to each other -- in this case it might be worth selling weapon upgrades on the freight cars to prevent them stealing kills. For this reason, I did this achievement last.