From #OpenTTDCoop

Mainlines (also called Axis) are the most important tracks in our cooperative games. Usually, they have two tracks in each direction and between them are at least three tiles of free space (LL___RR) - the more the better. This is mainly important, because it makes building Hub's and Slh's a lot easier (in recent games there were even 8 tiles in between the tracks).

• Mainlines are never directly connected with industries.
• Mainlines are connected with each other through Backbone Hubs (BBHs).
• Mainlines are connected with Sidelines through Sideline Hubs (SLHs).

Obstacles

Usually we play our games on Hilly maps and that means we landscape a lot for our mainlines to be straight. Unfortunately we often come across Transmitter towers and sometimes towns or industries. Towns can be removed (see Removing towns) but industries and transmitter towers can't. Therefore we often need to build around them or higher (or lower) the mainline down. Because we build networks that need to perform we have some guidelines for this.

Climbing

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When climbing up a hill (especially with the slower trains like monorail and railway trains) we use what I like to call 'stairs'. This means we go up, do a few squares (if we play with slower trains we take the train length. If there is room enough we take even more) flat and go up again. When climbing down a hill steps aren't really required but as we build the left side and the right side at the same level we do.

Around

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Ofcourse sometimes there is no room for moving the landscape up and down (for example: another industry or tower blocks it). On the picture here on the right is shown how we build around a transmitter tower. There was no room for the climbing method so we went around it instead. Notice that the track that is not build as the LL___RR rule must be big enough to fit the largest train running on the network on it.