Short term?
Maybe someone has the ability to code a way to 'attach' depots so that they act as one. We can build a few together so it looks good but they are still individual tiles....
Longer term?
The construction of GRF's that produce depot tiles. Id imagine this will be simpler if the above coding is possible - just like stations.
The above image includes a 'train washer' that Ive just drawn up as a rough idea of what we could aim for.
Website I was using as a reference:
Train Maintenance