Sometimes Trainz causes all kinds of issues when loading a route / session for the first time that can require reloading that same item twice to fully precache the said item. Therefore, it is highly recommended that you perform the prebuild prior to loading as this can resolve issues caused by not precached items.
 
The following applies to Trainz A New Era (TANE), Trainz Railroad Simulator 2019 (TRS19), Trainz Railroad Simulator 2022 (TRS22), Trainz Plus (Trainz +, TP), and newer versions.

Section 1: Enabling development tools
First things first you’ll need to enable developer tools (if you already have these enabled then you can skip to Section 2). To enable the developer tools:
1. go to Trainz Launcher > Settings > Dev tab > place a checkmark in the box next to ‘Enable advanced debug tools’
2. close Trainz completely
3. open Trainz again. Do not start the game.

Section 2: running the prebuild command
1.. go to Developer > Run TrainzUtil Command. A command box will appear.
2. type in without the quotes ‘prebuild’ into the box, then click on ‘OK’ or hit enter on your keyboard.
3. wait for the task to be completed. Depending where your Trainz content is installed (HDD or SSD) and on the amount of content you have installed this can anywhere from a few seconds to several minutes.
4. Close the log window that was created by the prebuild command and launch Trainz. You’ll also notice that Trainz runs faster as it doesn’t have to do so much precaching anymore unlike before.

More information about the TrainzUtil Command can be found on the Trainz wiki using the following  Link