Website Burkhard Jahnen
 

Railway design - Intersections

As you most probably already experienced, a wrong intersections design is a potential threat to your train services. That's because trains change direction there and thus cross other trains' paths. On badly designed intersections, even trains running anti-parallel to each other (with the intersection essentially being just another part of the standard dual track) may be slowed down! Since intersections and junctions are inevitable parts of your track layout, however, they should be optimized so that delays in train services are kept at a minimum. Let me point out again that

• emphasis is on keeping the trains going,
• not on shortest routes.

Although trains apparently have to travel longer, they gain from the design proposed below, because - on the whole - traffic jams occur seldom in spite of a high density of trains. One last introductory remark: In the following, we will deal with intersections. But notice that intersections can be turned into T-junctions simply by omitting one of the branches of dual tracks.

What are conventional designs for intersections then? Well, the basic approach is to let two perpendicular lines of dual track literally intersect. With signals placed properly, you get a total of 4  track tiles in the centre of the intersection (see picture 1). Obviously, the effect of such a layout is highly unsatisfactory: No matter what route a train takes, any train arriving a second later has to wait for the first train to completely occupy the first track block of its destination track. With several intersections placed in close proximity, you can easily imagine the high risk of irreversible traffic jams.

• simple to construct.
• rather useless for medium and high train density tracks.

Since even those trains that continue on the same dual track are hampered, the highest priority is to establish the track block concept in the vicinity of intersections (Read the 'Track' sub-section for explanations about the track block concept.). Well, the overkill way to do this is the famous cloverleaf design, depicted in picture 2. It has been adopted for trains from the large highway or Autobahn road crossings. The most important features of this layout are:

• Trains not changing the track run entirely unaffected from each other.

• Trains changing the track are slowed down for two reasons:

  • They have to cover longer distances (the ones turning left anyhow).
  • The dual tracks are built on two different levels.

• The trains encounter track blocks of the standard 4 tiles length throughout the intersection.

Take a look at the enlarged version of picture 2 to fully understand how the track block concept is maintained. Even those trains that change tracks are offered full protection from other traffic since they completely fit into full-length blocks. On other TTD websites, you sometimes find cloverleaf intersections smaller than the one shown above. But they lack signals placed sufficiently far apart so that the risk of traffic jams is drastically larger. One particular feature of this kind of intersections - no matter how far apart signals are - is that a train turning left has to travel longer than any other train: It first passes under or over the other track, then changes the level and finally travels in the wrong direction before it reaches the destination track. That's the price you pay. Besides this, cloverleaf intersections are

• too big and

• rather difficult to construct. In other words: time consuming. Time spent on constructing such a layout is wasted time.

So, your everyday crossing has to be considerably smaller and still has to let a high density of trains pass smoothly. I therefore developed two alternative layouts, one for medium density traffic and one for high density traffic. Both are based on the conventional intersection shown at the top of this page. In addition, the second layout is adopted for connecting terminus stations to the mesh of dual tracks, as has been put forward in the 'Tracks' sub-section.

Intersection for medium density traffic

Let us discuss the intersection for medium density traffic first. The basic version is shown in picture 3. As in the conventional design, both dual tracks intersect on the same level. However, the new design is different in that it is based on dual tracks three tiles wide, a concept proposed for entire dual tracks (take a look at the 'Tracks' sub-section). The centre part therefore consists of a square of 9  tiles so that the trains can be forced into a kind of 'roundabout'. Notice how the four signals make trains go in an anti-clockwise direction. Trains that remain on their tracks as well as those that turn right are unaffected by the roundabout. Those that turn left actually use the roundabout. Trains are even allowed a full U-turn, a feature that may lead to unnecessary traffic due to trains heaving 'lost' their destination or roving around on their way to a depot. So this is a disadvantage that comes about due to the fourfold symmetry of the intersection.

For the trains that leave the intersection, the first full-length track block already starts with one of the tiles in the centre. This way, trains turning left still occupy part of the track block they have come from when they are stopped in front of a red signal in the roundabout (see picture 4; notice the signal at the bottom in the left corner showing 'red'). Keep in mind that trains should consist of 5 or 6 wagons (I again refer to the 'Tracks' sub-section). Trains running straight across the intersection even have most of their wagons inside the track block they have come from when they are stopped in the centre. In both cases, trains are able to 'retreat' into their original track block, thereby making room for other trains to leave the intersection as planned. Of course, a train 'retreats' (i.e. reverses its direction) only after having waited in front of a red signal for a while. This means that traffic jams are not resolved immediately, but reliably. That's what we want. By the way, trains turning right either fully occupy their original block or completely make it into the destination block. They don't even have to retreat, because they always occupy well-defined track blocks.

You may have noticed that there are two consecutive signals on the exit tracks. They are a kind of additional security feature. Suppose we have two trains turning right, one after the other. If there was just one signal on the exit track, this would show 'red' after the first train has vanished into the second track block (the next block after the one that starts in the centre of the intersection). In some cases, depending on the overall track layout, the next train would travel in the wrong direction instead of turning right towards the red signal. Two consecutive signals effectively inhibit such a behaviour.

Picture 5 shows how mandatory servicing depots can be incorporated into the basic version of this intersection, making it even more traffic jam-proof. The layout shown here actually corresponds to the enhanced version of mandatory servicing depots which is introduced in the 'Servicing' sub-section. It results in only slightly longer distances trains have to travel in comparison to a situation without the depots. Placed at the entry tracks and close to the intersection, the depots also serve as areas of retreat, replacing the conventional last track blocks on the entry tracks. What's so nice about the depots is that they have a capacity of two ore more trains at a time! So they are actually superior to conventional track blocks since these have a capacity of just a single train. Really high density traffic thus turns itself into a low density one by trains retreating into depots. One tiny little detail is vitally important for this concept to work properly, viz. the second signal facing the compulsory signal on the track that runs past the depot. If this signal was missing, a train with 5 wagons that reverses its direction after an unsuccessful attempt on turning left at the intersection would not be forced into the depot (see picture 6)! Of course, trains with 6 wagons would not fail to enter the depot, because their last wagon would remain on the track tile leading to the depot.

Ok, traffic jams are resolved reliably. But are they avoided altogether? No, the design proposed above is for light and medium density traffic only, because two trains arriving from opposite directions at the same time that try to turn left already produce a traffic jam (see picture 7). So something had to be invented that makes this event less likely to happen.

Intersection for high density traffic

With this advanced design, four trains are 'needed' to produce the same traffic jam. Take a look at picture 8, where the version without mandatory servicing depots is shown. Trains turning left are forced onto the additional tracks (the yellow arrow in picture 8 highlights one of the four symmetrically equal routes). These trains now re-enter the intersection in a similar way as those trains that turn right into the same exit tracks (orange arrow in picture 8). Four of the pieces of track have to be removed from the centre of the intersections in order to make the trains take the new routes. In picture 9, a situation potentially leading to a traffic jam is shown. Please note again that such a traffic jam is resolved automatically and reliably since trains B and D can reverse their directions and fully retreat into the track blocks where they have come from!

Although this version of the proposed intersection design is more suitable for high density traffic than the version shown above, it basically doesn't consume more ground. When mandatory servicing is re-introduced, however, some extra track tiles are needed, as the previously propsosed track layout around the depots cannot be used. Besides this, slight changes to the additional tracks for trains turning left have to be made (see picture 10).

By the way, you can easily combine the advanced version with the basic version in that you omit some of the additional track tiles (e.g. because there are no trains turning left into some of the four branches of dual tracks). In that case, you can also switch back to the mandatory servicing design shown previously, where the trains have to cover a smaller distance. Let me also point out again that turning this intersection into a T-junction is fairly easy. You just have to know which of the new tracks belongs to which branch of dual tracks you don't need. Take a look at picture 11 for a lead. In fact, the situation shown in picture 11 closely resembles that of a terminus station next to dual tracks: Simply replace the other branch by a station of any size. This is discussed in slightly more detail in the 'Stations' sub-section.