This is a programming exercise based on the one presented here: https://github.com/gehel/exo-train
- Trains always "move" right-to-left. Nothing in the test cases suggests otherwise. So, the left-hand end of the string represents the front of the train in all cases.
- "Fill" always changes an entire freight car in a single go, not character-by-character. Cars are always filled front-to-back on the train.