BetterTrainBoarding

Unlock the peak efficiency of trains (and also metros) by fixing boarding behavior:

Passengers board trains (and metros) sensibly by each preferring the closest compartment
- To be explained below

Mod Status

No known compatibility problems
Absolutely save-game compatible: determining which CIM boards which trailer is an instantaneous operation

Because this mod is very small and simple, I am open-sourcing this to everyone out there.

Types of Transportation Covered

These transportation types are currently covered by this mod:

Anything under PassengerTrainAI:
- Trains
- Metros
Trams

Note: While not directly tested and unprovable from static analysis, monorails should also be covered as well. Do notify me if I am wrong.

These transportation types are not yet covered, but are at a low priority:

Buses
Trolleybuses

They are low priority because of their relatively small size compared to trains and metros. While they will have the same behavior when you utilize multi-trailer buses/trams etc, the effects are less obvious, and are negligible. At least for now.

Unless you are using mods that modify CIM's walking speed, but still, those mod would scale up boarding time quite fairly across all types of transportation, so ultimately the boarding time of e.g. buses are still "comparatively negligible".

Motivation

This mod aims to salvage vanilla CSL trains and metros from being an ineffective transport option to a reliable transport option. Supposedly, when buses and trams face throughput problems even when Express Bus Services (another mod) is used, then it is time to reorganize them into a high-throughput train/metro line.

I noticed how, for example, the metro becomes unreliable when a lot of passengers are waiting for it. Upon closer look, this is because the game is loading metros in an inefficient way, and in turn, metros spend too much time at stations simply to load passengers. This is unacceptable!

Technical Information

There are gifs at the Media directory to show the behavior difference between the base game and this mod.

We will now explain how this mod works in terms of the algorithm.

The base game

Train/metro begins loading passengers:

For each waiting passenger, from the front of the platform to the end of the platform:
    Find closest free trailer for the passenger
    If such trailer exists:
        Passenger enters that trailer
    Else:
        Entire train is full; stop this operation

This results in a negative feedback loop when there are so many passengers at a platform that the train/metro cannot take them all. Some passengers are left behind, then the next train arrives, then the passengers use the above behavior to board the train, which delays the train. When every station behaves like this, the throughput is greatly reduced. This can only be fully "fixed" only when the "passenger cluster" at the end of the platform is cleared.

Moreover, this way of loading passengers do not take into account the wait-timer of citizens, which makes passengers who waited too long and could not board the train utilize their "pocket car", making the situation worse.

This mod

Train/metro begins loading passengers:

For each waiting passenger, identify their ranked choices of preferred trailer order by distance
(ie, they prefer the trailer that is closest to them, and then the 2nd closest, ...)

Note: # of ranked choices = # of trailers that the train has
For each available rank: 
    Sort the ranked choices: passengers that have waited a long time will be prioritized
    Try to satisfy their choice by checking whether the chosen trailer is actually free:
        If free, then passenger enters that trailer
        Else try the next ranked choice

The negative feedback loop is greatly reduced. When the train/metro cannot load all the passengers at the platform, passengers who cannot board the train/metro are somewhat evenly distributed along the platform, and when there are space available, passengers can board the train/metro intelligently and efficiently. There may still be some passengers who need to move to a compartment far from where they are waiting the train/metro, but their number has been greatly reduced. This minimizes delay.

Moreover, this algorithm prioritizes those passengers that have waited too much time. This reduces usage of "pocket cars" among passengers.

Vectorial1024 / BetterTrainBoarding