Railway train controller for Unity3D -- 2

A 'basic' railway train controller is required for Unity3D. NO 3D assets are required. The controller should allow for a 3D object to be tied to and follow a 'rail' system. The controller should allow for believable though simple physics, for example inertia and weight. Also required of the controller are interactive abilities: - the ability to 'switch' tracks at the users command by way of a button. - the ability to add or remove further 3d models to the train consist (think carriages that can be coupled/ uncoupled). Ideally a weight and power system would be integrated. For example, the 'train' has 100 (units) power and weighs 25. Each carriage weighs 20. Adding carriages to the load will both slow progress and eventually exceed the quota. E.G. 2 carriages makes progress slow, 3 makes it slower still.