How would you keep a top view of a train on the tracks with the Box2D physics engine?

Question

I think it would be fun to model a top view of a train following a track, traversing switches and so on, using a physics library like Box2D. What joints and motors would I need to make this work?

I'm curious about how to implement the forces needed to make the car follow a spline track so it can bump into other train cars, pedestrians, DeLoreans etc. Just saying "the car is now at spline(t)" for each time step would create excessive forces in the physics engine. If I understand correctly, you have to stick the car onto the track with one force, constrain its angle to tend towards parallel with the track with another (or stick the front and back of the car to the track with two forces), and create another force to propel the train forward. I'm looking for some details on how to accomplish these things.

Answer 1

Why are you worried about keeping it "on the tracks"? Where is it going to go? Gravity should keep it down, object intersection should keep it up, and so the only directions you need to worry about are forward and backwards. That's where a motor comes in, and you're done. The rest is decorations.

In response to edit of problem:

Siderails. And have the train long enough / rigid enough compared to its width that you can navigate crossings (make them closer to right angles to minimize the crossing problems.

Answer 2

A top-down view (i.e. seeing the train from the sky) doesn't really require a 2d physics engine - if I understand you correctly. In fact, it seems like it wouldn't really help with the problem (if you want a train simulation), but then maybe you just wanna try it out for fun. :)

However, what about putting something like a slider joint on the train and the cars, and a motor on the locomotive. The slider joint might need some special implementation; you probably want to run the train along a spline and not a segment of straight lines, right?

Some sort of ball joint would connect the cars together.

Answer 3

I believe it would be easier without "real" physics, like the ball movement of games such as Luxor or Tumble Bugs. Meaning: let the train follow a spline which is defined by the tracks.

Using phyiscs is probably overkill to make a train follow a track and could lead to all kinds of undesired side-effects, including jerky motion, train derailing, train getting stuck on junctions, etc.

You could still join the individual wagons together using physic joints, however. Just make sure that only the locomotive gets acceleration forces, the rest of the train just follows or is pushed but stays on the spline.

Answer 4

The implementation is not so toughand I was able to prototype something in a few hours that does the basic job. It will require a lot of work to make it run smoothly, but it's essentially just "siderails."

Being top-down you obviously first must turn off gravity in Box2D. Second, build a train. Treat train wheels like car wheels and it'll suddenly get a lot more simple. For tracks you have a few choices:

• Create your own game object (not in the box2D world) that is a simple line the train will then "follow" (you can use motors on train wheels to "steer" towards the line). Then just overlay the line with some nice wide "rail" graphics and you have a nicely faked system. Tell the wheels to turn off if it strays too far from the line and presto, you have a derailment.

• Create actual physical rails - outside rails (like siderails) that the trains "wheels" will bump into. They will have to have gentle curves in this instance, which could be very difficult given the limited resources you have (simulating a nice slow curve out of boxes in Box2D is rough on the processor)

Then just let your train go!