This page is intended to explain a little about the DCS control system and what you can do with the remote handset(s) and Track Interface Unit(s).
Firstly it may be as well to explain what you can expect if you already have an electric power-supply/controller
All MTH gauge 1 locomotives can accept a conventional AC , constant voltage DC or rectified DC supply. To obtain a reasonable maximum speed and power the supply should be capable of a minimum of 5 amps at 18 volts. It should not exceed 22v, 20v is a "happy" high power value. If you like multiple unit lash-ups, double heading or whatever you might want to consider a 10A supply. On a conventional resistive or variable voltage supply you can get forwards and reverse motion, lights basic sound including start-up, running and shut-down sequences along with smoke output. The upper volume of smoke and sound can be limited by potentiometers on the locomotives. Those with an AC output controller may be able to send coded power pulses to reverse the locomotive, sound the horn or bell or loosen the couplers. If your power supply generates a high frequency "chopped" output then the power pulses may be interpreted as illogical DCS commands and for this reason "chopper" controllers are simply not recommended.
So what can the TIU-handset combination do? Let us consider the general specification of the TIU:
The TIU has four power inputs and four power outputs. Two of the input/output units can only be used to control DCS equipped locomotives. These outputs are effectively fixed voltage with command signals being sent down one of the rails. Just for your information the right hand side of the locomotive is the one that picks up the signal so if the locomotive does not seem to respond try swapping the power leads over (this will also need to be attended to if you have a reversing loop).
The other two outputs are factory ready to be used as either independent "fixed output" DCS control mode or for conventional (variable voltage a/c) control. The addition of a simple bridge rectifier to these two outputs allows the TIU to control normal dc locomotives via variable voltage rectified dc. The features that are available depend on whether the the motor in the locomotive needs an ac or dc supply. If you happen to already have a conventional dc transformer/controller then the TIU can "pass through" a variable voltage but will not act as the voltage controller itself.
Each output controls a "loop" or section of track. These track sections must be electrically isolated from each other and common-return wiring is not an option. Under DCS control more than one locomotive can operate on each track section, either as a diesel lash-up, double heading, push-pull helper or independent trains as long as the total power drain of 10 amps per output is not exceeded. If you had four track sections and your locomotives cruised at an average of 3 amps then one TIU could deal with 12 seperate running locomotives and any number of others parked up. Obviously the TIU cannot provide more output power than it is supplied with and each of the outputs in use will need an adequate supply on the input side. If you tend to operate only one locomotive per output then a 5 amp supply will most likely suffice for general purposes with 10 amp capability recommended if you want maximum flexibility. 18 volts should be adequate, and will lead to longer service life on items like the locomotive and coach lighting, but if you really want to test for maximum drawbar pull then 20 or 22 volts will give roughly 10% and 20% more effort. MTH recommend that a 15 amp quick-blow fuse is used on each of the TIU inputs, to protect against the possibility of damage if there is a short on the track, and it is perhaps worth repeating that 22volts is the maximum voltage allowed on the supply. The TIU can be connected to a chain of Accessory Interface Units and also to a computer to allow new sound files to be loaded into locomotives. It should be noted that there are no manual controls on the TIU itself (other than an emergency stop panic button) it can only be controlled by the remote handset.
The handset is powered by AAA batteries and has a free uninterrupted range of around 50 feet. Remember that the range is not from the handset to the locomotive but from the handset to the TIU. The handset has a two-line lcd display, a central thumbwheel and an array of pushbuttons that give immediate control over the bell, whistle/horn and coupler without you having to remember the code sequence for these particular features. MTH locomotives that are on controlled sections of track are first detected and then logged on to the handset's memory. The selected locomotives can then be controlled for speed, direction, sound and smoke volume, all the bells and whistles and other sundry sound effects that the locomotive has been programmed with plus the uncoupler for those recent locomotives that have this feature. The handset can be set to one of five frequencies should you want to operate layouts in close proximity at an exhibition. Iif you want to control more than four track sections with further TIUs they can be set for common control, in this instance each TIU will broadcast the control signals and it will not matter which section the chosen locomotive happens to be on. It is also possible to have two operators, each with a handset, controlling through the same TIU. In this instance, if there is a conflict in operator intentions, the locomotive(s) will obey the most recent valid instructions.
If there is a remaining bugbear with DCS it is the controller only puts out command signals on one wire and that the locomotive only picks up the command signals on one rail which means that it is possible to have the locomotive on the track "the wrong way". 2-rail Protosound 2 locomotives have a polarity switch to vercome this but, if you have a reversing loop you may want to fit a dpdt switch to the track feed or simply swap the power leads if such a simple solution is acceptable. In a perfect world the next generation of TIU would send signals down both rails and also provide variable dc control straight out of the box. [please note that DCS was developed around an O gauge 3-rail heritage and that the Protosound 3 decoder, as fitted to the Ho locomotives, can receive on both sides]
The TIU and handset tot up to around two hundred pounds and as such may not be considered a bargain in the first instance. I would however suggest that you look at the pricing of just one conventional transformer-controller that can actually manage a 10 amp output and you may find that you are looking at anywhere between one to four hundred pounds. If you then consider that the TIU needs just a basic 18v ac supply, that it can control four separate outputs and gives you full access to all the DCS control features then it seems a far better deal in comparison.
For further notes on the TIU regarding capabilities, wrinkles and potential pitfalls on lengthy track sections, outdoor tracks and suchlike I would suggest taking a look at the information kindly supplied by Rayman4449 .
I would also draw your attention to the DCS Commander which, if it's single 5 amp maximum output is sufficient, may provide a cheaper option and also the more recent DCS Commander Remote (which is even cheaper but which does not operate all the minor controls).