Table of Contents

Train Rendering Example

Based on the existing MTR model

You can simply take the appearance of an existing model and add additional rendering logic using JS. It's probably easier to do it that way.

Complete rendering control with JS

It's also possible to use base_type and not use MTR's built-in model drawing process at all, instead using JS to control it all.

The following code implements loading the OBJ model, selecting groups to display as needed to achieve an effect similar to the original MTR train. Feel free to copy it and modify it.

The following models are used: groups body, head, end, headlight, taillight, where head, headlight, taillight are in the Z- direction and end is in the Z+ direction; and groups doorXNZN, doorXNZP, doorXPZN, doorXPZP, doorlightXN, doorlightXP for doors and their indicators. 

// Load groups from train.obj as Map of RawModel
var rawModels = ModelManager.loadPartedRawModel(Resources.manager(), Resources.idRelative("train.obj"), null);
// To upload all the RawModels, I wrote an uploadPartedModels to upload each parted model in the Map one by one.
var models = uploadPartedModels(rawModels);
// Use this texture for gangways
var idTexConnector = Resources.idRelative("connector.png");
 
// We don't need logic in create and dispose, so we don't need to use them.
 
function render(ctx, state, train) {
  let matrices = new Matrices();
 
  // Process each car in turn
  for (i = 0; i < train.trainCars(); i++) {
    // Draw the front and rear ends of the car as desired, as well as headlights or taillights
    matrices.pushPose();
    if (train.trainCars() == 1) { // There's only one car in total, showing a double-ended wagon.
      matrices.rotateY(Math.PI);
      ctx.drawCarModel(models["head"], i, matrices);
      ctx.drawCarModel(train.isReversed() ? models["taillight"] : models["headlight"], i, matrices);
      matrices.popPushPose();
      ctx.drawCarModel(models["head"], i, matrices);
      ctx.drawCarModel(train.isReversed() ? models["headlight"] : models["taillight"], i, matrices);
    } else if (i == 0) { // This is the first car whose head should be in the Z+ direction and whose end should be in the Z- direction, so flip it 180°
      matrices.rotateY(Math.PI);
      ctx.drawCarModel(models["head"], i, matrices);
      ctx.drawCarModel(train.isReversed() ? models["headlight"] : models["taillight"], i, matrices);
      ctx.drawCarModel(models["end"], i, matrices);
      matrices.popPushPose();
    } else if (i == train.trainCars() - 1) { // This is the last car, with the head in the Z- direction and the end in the Z+ direction, so we don't flip it
      ctx.drawCarModel(models["head"], i, matrices);
      ctx.drawCarModel(train.isReversed() ? models["taillight"] : models["headlight"], i, matrices);
      ctx.drawCarModel(models["end"], i, matrices);
    } else { // Intermediate car, draw two ends
      matrices.rotateY(Math.PI);
      ctx.drawCarModel(models["end"], i, matrices);
      matrices.popPushPose();
      ctx.drawCarModel(models["end"], i, matrices);
    }
    matrices.popPose();
 
    // Drawing the body
    ctx.drawCarModel(models["body"], i, null);
 
    // Drawing door indicators
    if (train.doorLeftOpen[i] && train.doorValue() > 0) {
        ctx.drawCarModel(models["doorlightXP"], i, null);
    }
    if (train.doorRightOpen[i] && train.doorValue() > 0) {
        ctx.drawCarModel(models["doorlightXN"], i, null);
    }
 
    // Drawing doors
    let doorX = smoothEnds(0, 0.81, 0, 0.5, train.doorValue());
    let doorXP = train.doorLeftOpen[i] ? doorX * 0.81 : 0;
    let doorXN = train.doorRightOpen[i] ? doorX * 0.81 : 0;
    matrices.pushPose();
    matrices.translate(0, 0, -doorXN);
    ctx.drawCarModel(models["doorXNZN"], i, matrices);
    matrices.popPushPose();
    matrices.translate(0, 0, doorXN);
    ctx.drawCarModel(models["doorXNZP"], i, matrices);
    matrices.popPushPose();
    matrices.translate(0, 0, -doorXP);
    ctx.drawCarModel(models["doorXPZN"], i, matrices);
    matrices.popPushPose();
    matrices.translate(0, 0, doorXP);
    ctx.drawCarModel(models["doorXPZP"], i, matrices);
    matrices.popPose();
  }
 
  // Drawing gangways
  for (i = 0; i < train.trainCars() - 1; i++) {
    ctx.drawConnStretchTexture(idTexConnector, i);
  }
}
 
// Uoload each RawModel from the Map obtained from loadRawModels
function uploadPartedModels(rawModels) {
  let result = {};
  for (it = rawModels.entrySet().iterator(); it.hasNext(); ) {
    entry = it.next();
    entry.getValue().applyUVMirror(false, true);
    result[entry.getKey()] = ModelManager.uploadVertArrays(entry.getValue());
  }
  return result;
}
 
// Door animation copied from MTR
function smoothEnds(startValue, endValue, startTime, endTime, time) {
    if (time < startTime) return startValue;
    if (time > endTime) return endValue;
    let timeChange = endTime - startTime;
    let valueChange = endValue - startValue;
    return valueChange * (1 - Math.cos(Math.PI * (time - startTime) / timeChange)) / 2 + startValue;
}

Source