I love the aesthetics, it gives me wasteland/alien-planet vibes, very polished, love the loopable music and sounds in general besides being subtle.
This can be expanded a lot, not exactly only "more enemies" which is obvious, but a more roguelike mechanic, different train types (sort of a class like behaviour), choice mechanics like upgrade trees after X experience, or you can choose where to go if you shoot a junction lever where you have two different ways, different biomes.
I don't find much meaning on braking the train, maybe I was missing something about it, the only way I can see it was useful for me is not missing a chest when I have cooldowns.
EDIT - Stuff I forgot to mention but I took note in my head:
I don't think there is a problem with the contrast of the game besides not having much detail, the only way I see you can fix readability is with a closer camera look , but not too close it loses the wasteland aesthetic, maybe introduce fog so you don't have any reason to raise the camera.
Controls are a little bit weird, I don't see how that could be improved, but I like the different options between using A and D or using numbers, if you didn't plan on "expanding" the train, you could use certain button as class buttons, like Q,W,E,R... which have a meaning not just "left and right".
Wow, I really appreciate the amount of time you put in your comment. Thank you so much for this detailed feedback.
On your first point, I completely agree with your thoughts on expandability. I had started to prototype a shield generator car and a laser train, but disabled them before uploading to meet the competition deadline. I love the idea of junctions that you can shoot a lever to choose which path you go down. I think that adds a lot of meaningful choice in how the player progresses, not just coasting on one rail the entire game. It is funny, but if you are able to survive long enough in the current build (~60 minutes) you will reach the extents of the game and reach the null void where the background becomes grey. It is actually kind of a cool feeling going between those two zones, even if it wasn't intentional, so I think your idea of biomes is spot on and definitely something that should be pursued.
You also make a good point about the train braking. Early in development I found it useful so I could not miss an item chest, but as difficulty was tuned more and enemies were added, it became meaningless as you mention. My current build of the game I've actually already replaced it with a 10% self heal that seems to feel a lot better, and helps smooth out some of the difficulty spikes when huge waves are spawned.
With regards to detail/contrast, I'm a programmer by trade so all of the art assets I created were a best effort attempt while trying to minimize the chance someone could notice my drawing errors lol. I think I'm slowly improving, but I definitely recognize I have a lot of room to grow, and a more complete version of the game I would probably want to contract a real artist to replace the assets I had made. I do like the idea of introducing fog, and more particle effects in general would probably help sell the atmosphere while also bounding the camera extents as you mentioned.
On your last point, I had originally planned on having the train be expanded over time, say after every boss fight you could get a new train car added. That obviously was cut for competition reasons, but it is still something I am playing with. Early versions I had actually been testing with trains that were 20 cars long and it felt pretty awesome, but it definitely becomes even more difficult to control. I really do appreciate folks feedback with regards to the control scheme and this is something I am thinking very seriously about to try to resolve.
Wow, the thing about getting out of the map into the null void is awesome, I can see it as an easter egg, a more polished null void (like intentional) that could happen after certain kilometers. Also about the kilometers, maybe there could be more reasons to "brake" or "go fast" than just advance into the game, it will be really hard to stick decisions into a linear setup, but for example there is stuff you can use at your advantage: more difficult as you progress in time (so enemies scale with time like risk of rain 2), that is: unique enemies for each time frame, with modifiers.
Enemies would be the hardest task as you need to consider how many dimensions you have to parameterize, a fact is that how many permutations on that dimension doesn't mean how many unique enemies, so you could make some sort of 5 dimensions for the enemies that makes sense to combine when applied to unique enemies (if you kill them too close they explode and damage your train, enemies that are invulnerable until you kill its minions, etc) those are stuff you can combine with unique enemies.
it can easily fit the "spam enemies" fest and chain reactions, but I think the "bleak" atmosphere is a nice touch.
So your progression system can be based mostly on shooting a lever to change the path gives a lot of intention into the game, but at the same time keeps it linear enough to feel constrained, but that doesn't mean you cannot brainstorm like for example one of the paths leads to a portal at some point, your idea has a lot of expressions to make it progressive, you start on a more "realistic" and wasteland set until you advance into the game, even adding stations to buy stuff.
About the idea of expanding your train, I think this one would be more fitting for a "barotrauma" style, which is having a set of different looking submarines in that case (here would be unique train combinations) each with it's advantages, for example one could be more tanky but slow, others more fast but cannot have a certain weapon (like heavy damage weapons) and how that do an effect on future decisions. Technically you can expand the weapons (like slots) but not the train itself.
Regarding Artstyle I was imagining while playing, a washed palette, sort of 3/4 perspective and not too clean (somewhat noisy), it would be sort of a factorio train for example:
Good luck on this game if you decide on expand it, I would've love to help you on the art direction but I'm just a pixel artist.