Bi-Directional Roll-On, Roll-Off Stations

-->currently revising this page completely, right now I would consider this at best a rough draft<--

Why Bi-Directional Ro-Ro (BDRR) stations?

A big advantage of BDRR stations over traditional ro-ro layouts is that looping traffic around to one side is not required. When building a station relatively close to the ML all that needs be done is to get traffic from the far side of the ML across the near side. There is no need for trains from one direction to make a 180° turn before entering, or after exiting the station. Stations can be built so that the only complete reversal of the direction a train is traveling in occurs in the platforms, provided this ability is enabled in the difficulty settings, which for #ottdcoop games it is. This allows for the stations to be built closer to the ML allowing for shorter traveling time from ML through the station and back to the ML.

However, BDRR stations are not well suited to every situation. They would be very wasteful if built along a one way ML. They typically have a more complex track layouts than traditional ro-ro stations as there is an entrance and an exit on both sides of the platforms. It is more difficult to make every platform available to every entering tack. For a given number of platforms, a BDRR station will almost always take up more space than a ro-ro. Trains must be entering and exiting from different directions, as in a secondary industry drop station. Generally at industry pickup stations trains are arriving and departing a uniform direction, a BDRR in these cases would be needlessly complicated.

Throughout this page I am choosing to ignore layouts that allow entering and exiting traffic from different platforms to interfere with one another, so no evil-x's or mess-o-track PBS layouts.

Considerations when building a BDRR

The three most important things to consider when building a BDRR are:

• number of platforms
• train length
• curve length

When choosing the number of platforms, there must be enough to meet the demands of the trains using the station, but too many creates a very bloated station. Under ideal circumstances I like to have five platforms per entrance, as five platforms are generally enough to meet the demands of a single entry delivering trains at full capacity into a station. However as this is rarely the case, five platforms per entry is generally not needed.

Train length is important in its own right, and not just a factor in curve length. Since trains suffer a penalty when encountering three or more turns in one train length (insert picture to illustrate), attention must be paid to this. With how tight I like to build, I use a section of track TL-1 in length to allow trains to straighten out before entering another curve. Although this will not apply in every case, it may be possible to use shorter straight sections.

Curve length is obvious and commonly dealt with in #openttdcoop games. As it applies to BDRR layouts, I find myself having to make modifications to accommodate train length more often than curve length. Be sure to built a test track and test the curve length with the actual engine and wagon combination(s) being used, as curve length is often less than TL.

When building a station to serve two different train lengths, build to the longest TL and the longest CL, they may not be for the same train.

Ideally it is best to split entering and exiting trains as soon as close to the platform as possible. The quicker this can be accomplished, the quicker the platform is ready for the next train.

Pre-Signals vs Path Signals

This is one instance where I feel path signals have a distinct advantage over pre-signals. With pre-signals it is possible, and likely, that trains entering from both sides of the station will choose the same platform. This means one of them will get to the station, and the other will be stuck in the pre-signals waiting for a green light. It is possible that a train stuck in the pre-signals will block access to all platforms from it's side of the station. To avoid this waiting spaces can be used, built so that trains are choosing waiting spaces, not platforms (put a regular block signal before the station). This can add considerable length to the station. Using path signals has the advantage that trains not only choose platforms, but reserve them as well. Every train passing the path signal has a platform, a train from the other side cannot "steal" it. One important design consideration with path signals is to keep the distance from signal to platform as short as possible. This is because a platform is reserved for the entire time it takes for the train to travel from the path signal to the platform, then out of the platform again. Trains should get in and out of shared track blocks as quickly as possible to make way for the next train.

Inherent choice vs load balanced

I am using the term inherent choice to describe a station where each entry has a built in choice of every platform. This can be a daunting task with BDRR stations as the stations become very large, very quickly. Generally I have been using sets of smaller station building blocks, and using a load balancer to distribute trains among the station sets. An example is in the current PSG #127 at the coal drop station. The ML in this game is LLL_RRR, so I built three sets of three platforms and used a 3to3 load balancer in front of each entry. This worked well for a time, but 154 trains is proving more than 9 platforms can handle. I am currently expanding this to three sets of 4 platforms and am able to reuse the majority of the existing entry and exit track. I have tested this layout on this map (in single player) and expect it to handle somewhere between 175-200 trains just fine. Expansion to 3 sets of 5 will require a major overhaul of the layout.

Relationship with terminus stations

BDRR layouts and terminus stations are closely related. If you take a terminus station and mirror it about the platforms, you have a BDRR. However, it may not be the most efficient BDRR. I consider the Osai style terminus station to be a very good terminus station, but an inefficient BDRR. (pic)(explanation)

Gallery and explanation of some BDRR building blocks I have come up with

2 platform building block: (pic) (explanation and key points)

3 platform building block: (pic) (explanation and key points)

4 platform building block: (pic) (explanation and key points)

5 platform building block: (pic) (explanation and key points)

2 entry/2exit per side inherent choice BDRR station (pic) (explanation and key points)