Flatland Competition 2020: MAPF and MARL for Efficient Train Coordination on a Grid World

TL;DR

This paper discusses the Flatland competition focused on train coordination using MAPF and MARL, highlighting innovative approaches and solutions for real-time scheduling in complex railway networks.

Contribution

It introduces the competition setup, presents four top solutions, and compares different MARL and MAPF strategies for train coordination in a simplified grid environment.

Findings

Graph-based environment representations improved coordination

Communication and prioritization mechanisms enhanced agent collaboration

MARL approaches showed promising results in train scheduling

Abstract

The Flatland competition aimed at finding novel approaches to solve the vehicle re-scheduling problem (VRSP). The VRSP is concerned with scheduling trips in traffic networks and the re-scheduling of vehicles when disruptions occur, for example the breakdown of a vehicle. While solving the VRSP in various settings has been an active area in operations research (OR) for decades, the ever-growing complexity of modern railway networks makes dynamic real-time scheduling of traffic virtually impossible. Recently, multi-agent reinforcement learning (MARL) has successfully tackled challenging tasks where many agents need to be coordinated, such as multiplayer video games. However, the coordination of hundreds of agents in a real-life setting like a railway network remains challenging and the Flatland environment used for the competition models these real-world properties in a simplified manner. Submissions had to bring as many trains (agents) to their target stations in as little time as possible. While the best submissions were in the OR category, participants found many promising MARL approaches. Using both centralized and decentralized learning based approaches, top submissions used graph representations of the environment to construct tree-based observations. Further, different coordination mechanisms were implemented, such as communication and prioritization between agents. This paper presents the competition setup, four outstanding solutions to the competition, and a cross-comparison between them.