HD Terrain requires TRS22-Platinum Edition or TRS-Plus.
In Trainz-PE, it's read-only to allow HD-routes created in Plus to load in TRS-PE. Editing an HD route in TRS-PE converts it to a 5-meter grid terrain from the high-resolution terrain.
Is it worth it?
That's up to you, and whether your computer can handle HD-terrain because that takes up a slog of memory and video processing power in addition to massive amounts of hard disk space.