Triggers to reverse the track feed for the analogue unit (that's how I was planning on doing mine) but how does the digital work? Sounds interesting if it can accelerate and decelerate rather than 'instant stop/start
Analogue shuttle units work by having a timer which reverses polarity after a set time.
The diodes that are in a gap at each end of the track (on one rail) only let the power go one way.
When a loco crosses at one end it stops (due to the one way diode) until the the shuttle unit reverses polarity.
Then the loco (being DC 'reverses' back until it crosses the other diode gap and it stops again waiting for the next timed reversal of polarity to go 'forward'.
The timer time includes the time for the loco to go from one end to another so it has to be set for total journey time between the gaps.
On the Massoth Dimax system, DCC shuttles can be done by using a feedback module which has magnetic sensors in the track (at either end or also in the middle) and the module needs to be programmed to do what it needs to.
The Tam Valley DCC shuttle unit works totally independently from a powered central station unit.
The loco decoder will have the 'soft' start and stop already programmed into the decoder for 'normal' use so this will still happen on the shuttle.
The TV shuttle also has timers which set the journey/wait time (or a pause in the middle somewhere with another 'triggering' gap).
The rail gaps, one at each end (and the option of one in the middle) are on the same rail (like the DC ones) but do not use diodes.
The loco will stop (with the already programmed deceleration) and wait for the unit timer to tell the DCC signal to command it to reverse back (again with the 'soft' accelerated start) till it crosses the other gap where a second timer will give the DCC command for it to go back again etc etc.
The two timers (in fact there are three - one for the middle stop) can be set independently for long or short stops etc.
The unit also has a speed control so that the loco speed can be altered (of course you could not use your system handset as the TV shuttle is an independent system)
DCC locos can be used on a DC shuttle (but be careful of on board 'keep alive' capacitors)
DC locos cannot be used on a Tam Valley DCC shuttle