Use Multiplayer Surveyor
Multiplayer Surveyor (MPS) is a new Trainz Feature for TRS2019, TrainzPlus and Gold Class members. Introduced in November 2020 (see Tony Hilliam on "Now Enjoy Trainz With Friends"), MPS allows multiple players to collaborate on building a route stored on a cloud server. In short, users can edit a route and operate trains on that route together in real time. Detailed information on this new Trainz feature can be found at Multiplayer Surveyor.
The MPS experience differs from standard (on your computer) Trainz in several important ways. Available routes are shown on the bottom menu bar with purple titles and sorted to the far right. The number of routes displayed is limited and you can remove any by selecting and then clicking on "ABANDON ROUTE". A green check mark indicates a MPS route you have joined previously. Selecting and clicking "EDIT ROUTE" will log you onto the cloud server and the selected route will slowly develop on your screen in surveyor mode. You may be placed in a queue in 94th place, but this is not real and re-applying will likely get you in to that route immediately.
There are no separate sessions; changes you (or others) make occur in real time, so the distinction between route editing and session operations is meaningless. You can switch to driver mode (TOOLS MENU) at any time. A red box appearing on your screen simply means your changes in that specific area are being processed—hold off on further editing for a moment.
Access to assets to develop the route is limited to a fixed content set, which is periodically expanded.
Related Tutorials and Guides to Using Trainz