Let's get some important details out of the way at the start: TSW5: Cargo Line Volume 4 requires the core game (currently on Game Pass) and either West Coast Main Line: Preston - Carlisle or the Great Western Express DLC to run.
Dovetail Games' Cargo Line Vol. 4 is a DLC for Train Sim World 5 that focuses on the transportation of military vehicles. Developed by Skyhook Games, the pack introduces a new type of freight wagon, the KWA Warwell, and offers new scenarios and timetable services for the two previously mentioned existing routes.
The main draw of this DLC is the introduction of the KWA Warwell wagons. These wagons, which have been a part of the UK rail network since WWII, are designed with a low profile to accommodate large equipment while staying within the country's restrictive loading gauge. The pack includes the wagons in three variants: empty, loaded with a generic tracked APC (Armed Personnel Carrier), and loaded with a generic 3-axle MRAP (Mine-Resistant Ambush Protected) vehicle. It is worth noting that, for licensing and thematic reasons, the vehicles themselves are not modeled with weapons or identification markings.
With Great Western Express you can take on new military movements, hauling equipment between Reading and Acton Main Line. These scenarios use a DB Cargo-liveried Class 66 locomotive, which is also included in the pack, the KWA Warwell wagons loaded with MRAPs.
On the West Coast Main Line: Preston - Carlisle the DLC adds military timetable services that reflect late 1980s operations, with the included Class 47 locomotives from the base route hauling APCs to Shap. The new wagons and the Class 66 are also compatible with the Livery Designer, Formation Designer and Free Roam modes, allowing for a good degree of customizability and replayability.
Cargo Line Vol. 4 is an interesting string to TSW's bow, and I wouldn't be surprised to see a US version with M1 Abrams, M3 Bradley and HummVee lookalikes "down the line" (sorry.) If the theme appeals and you already own the aforementioned routes then this is definitely a worthwhile purchase, especially at a price of only £9.99 (£8.99 with Game Pass.)
Many thanks to Dovetail Games and Indigo Pearl