Monorepos are single source code repositories that contain multiple independent components, which can be deployed using Railway's features for isolated and shared monorepo applications. Isolated monorepo components can be configured to deploy only a specific subdirectory of the repository by setting the "Root Directory" trigger. Shared monorepo components, on the other hand, require a global build command that compiles all components and dependencies together, and Railway provides a Custom Start Command feature for deploying these types of applications. This allows monorepos to be deployed to multiple Railway projects with different components running in each one.