MTH likes adding a "proto-" prefix to the trade names that it gives to various products and operating features.
Protosound 2: The locomotive will be fitted with a DCS decoder and the MTH sound system.
Protosound 3: Adds the ability to decode the majority of DCC commands to the DCS decoder (and also has the sound system). Originally developed for the Ho range it is now appearing in new Premier O gauge locomotives.
Protosmoke: The locomotive is fitted with the MTH smoke generator (and will most likely have a DCS decoder), for steam locomotives the puffs are timed to the driving wheels.
Protocoupler: The locomotive will have a solenoid activated uncoupling action on at least one coupler, this can be activated to effect an uncouple with sound effects via DCS or a pulsed a/c sequence command.
If you have not read about DCS elsewhere then it may be worth pointing out that a locomotive fitted with a DCS decoder does not have to be controlled by a digital controller, they can also operate from a traditional variable voltage controller and provide a decent range of default sound and/or smoke effects.
But what of Protoscale? This protoscale tag is now being applied to the majority of the new locomotives in the more detailed Premier range of O gauge models, all the european prototype locomotives have this feature as do the more recent american designs and it simply denotes that the locomotive is readily converted between a 2-rail or 3-rail pickup configuration. Hi-Rail locomotive versions are supplied in 3 rail configuration with the detachable centre rail rollers already fitted whilst scale wheel versions are despatched configured for 2 rail operation but with these pick-ups ready to attach.
Having received a couple of queries as to exactly how quick and easy (or otherwise) it is to convert from one mode to the other (MTH does not demonstrate this on their website so far, perhaps because they consider it so simple as to be redundant) I thought it may be for the best if I gave some additional information here. I would say that when you have examined the components it should be rather obvious as to what is required (and if not then you have the manual to assist) but for the sake of clarity the arrangement for the SNCF pacific is shown and described here. Pairs of sprung roller pick-ups, square plastic locating washers and attaching screws are provided in a resealable bag and these may be fitted to two threaded metal posts that are on the underside of the locomotive chassis. All you have to do is carefully turn the locomotive on it's back or side and attach the rollers after which merely flicking a small slideswitch on the underside of the tender body completes the operation. The black plastic locating washer fits between the base of the metal roller unit and the chassis and there is only one way that it can properly fit the chassis and pick-up and when attached it will prevent the roller arm from swinging sideways. With everything in position this should take less than a minute whether mounting or removing the rollers. Obviously the rollers are intended for use with a central rail but, for those that have the stud method of contact, it is perfectly possible to solder a metal skate to the leading roller with the trailing roller providing the rear contact force. On this locomotive the mounting holes are 62mm apart, the contact points for the rollers are around 94mm apart and a skate of around 110mm length should just squeeze in, which I hope would allow for at least three studs in contact. This may not be the very best, quickest or easiest method of converting back and forth, I suppose MTH could have arranged for twin rollers attached by a single screw, provided a skate as an additional item or arranged for a permanently attached central pick-up to raise and lower under digital control complete with some Anderson type launch sequence music from the sound system, but they have certainly done all of the hard work and I am not aware of any other commercial models that convert with such ease.
This is the arrangement for the Chapelon Pacific with the rear trailing roller fitted. The black mounting washers have pegs which engage in holes inboard of the threaded metal mounting posts and the front pick-up would normally be mounted so that the drop arm is attached with the roller leading. If the centre pick-ups were to be converted to a skate then it would only require another locating hole to be drilled and then both arms could be attached with a trailing (or leading if you prefer) action making it easier to give the skate a parallel action if that aspect is desired.