Bridges and Overpasses: Instructions

This page describes how to use new features in TrackLayer 2.65 to set up clipping for tunnels, bridges, and overpasses.

Tunnel Entrances

A tunnel entrance is a point where a visible track meets a hidden one.  When a car moves across this point, it is split into two display styles.  The split occurs at the point where the tracks join, and is normally perpendicular to the car axis, as shown in the upper diagram:

If the train is entering a tunnel with an angled portal, perpendicular won't work.  In TL 2.56 and above, you can now specify the angle of the split, as shown in the lower diagram:

To specify a split angle:

1.  Draw a short section of track at the angle you want, passing through the junction point.  The length doesn't matter, but it must be a single section unconnected to any other track.

2.  Right-click the section and mark it Tunnel.  It displays as a dotted grey line.

The program does the rest -- if it finds a section of tunnel track at a tunnel entrance, it clips the car accordingly.

Note: tracks marked Tunnel are visible only when you are using the Edit tool or the Track tool.
 

Hidden Regions

Hidden track is ok for long tunnels, but is not adequate for short underpasses or multi-level intersections.  To handle these, 2.65 introduces a new device called a clip box.  This allows you to specify a custom-shaped region where the train is to be hidden altogether.  We thank Peter Lloyd-Lee for proposing this concept.

A clip box consists of exactly four track sections, marked Tunnel, joined in a loop, surrounding some length of track.  When a train passes through, it is clipped to be invisible in the interior of the box, as shown here:

An application of a clip box might be when a train passes under a building or road.

To create a clip box on a single track:

1.  Draw four track sections surrounding the area to be clipped.  These must join to each other but nothing else, and must form a convex loop.

2.  Mark all four sections as Tunnel.  The quickest way is to shift-click and select all sections, then right-click and choose Tunnel. 

The sections change to dotted grey lines (or blue, while selected).

3.  Use View > Refresh to process the new box and begin clipping to it.  (Clip boxes are processed automatically when you open a layout, so this step is only necessary after drawing a new one.)

4.  Run a train through the box and watch it disappear neatly.

Multi-Level Regions

If a clip box surrounds portions of more than one track, then trains will be hidden on all of them.  For bridges and overpasses, this is no good, as it hides trains going across the top.  To handle this case, there is a trick you can use to disable a clip box for trains travelling on certain tracks.

The trick is this: if a track which crosses a clip box has a junction lying on one edge of the box, then a train travelling on that track will be visible, not clipped.  The following diagram shows an example.  The N-S track has a junction (green square) just at the top edge of the box, so this track is taken as the topmost one, and trains on it will be visible.

To create a multi-level clip box:

1.  Create a clip box as described previously, but surrounding more than one section of track.

2.  In the track you wish to have on top, introduce a junction where the track crosses either edge of the clip box.

This is tricky.  Normally to introduce a junction, you use the "track chopping" method described in the manual.  If you do this at a point where two tracks cross, you end up with a four-way switch, which is not what you want.  We may change this behavior, but for now, here is the best way to get around the problem:

2a.  Right-click the (dotted grey) edge of the clip box where you want to put the junction, and choose Untouchable.  This causes that edge to become inactive and unselectable while you create the junction.

If you later need to be able to select that edge again, right-click on any other track and choose All Retouchable.

2b.  Now chop.  Draw a temporary length of straight track from some empty spot to the crossing point where you want the junction.  This creates a switch.  Then delete the length you just drew.  This removes the switch and the temporary track, but leaves a junction.

3.  Use View > Refresh to process the box and update its clipping characteristics.

4.  Run trains over and under, behold your mods.

Note: this trick has limitations:  (a) the topmost track must be a single section going across the clip box -- results are unpredictable if you have a multi-section track path across the box;  (b) a clip box must not have more than one junction lying anywhere on its perimeter; (c) a clip box can have only one topmost track -- if you want two visible tracks crossing a box, you will need to split it into two separate clip boxes.