Dear Doc, Modellers all,
Glad to see the "Reversing loop" technique is working for you. The original testbed was/is HOn30, with 4" radii curves, PECO setrack turnouts,
and nothing but a 9volt battery for a throttle/powersupply.
As long as the "constant direction of travel round the loops" is not an issue, there is absolutely no need for any form of active detectors, reed switches, Infra red, switching relays, or anything else. Indeed, if your rollingstock wheel specs are up to scratch, you can even shove a train "cars first" thru the loops and it will still work!
(Although, depending on the position of the "stop block" on the exit side of the loop, the train may be fouling the turnout when the loco stops).
I have played with, and am a big fan of, long delay "dumb timers" for simply back-n-forth train/animation control,
esp to give additional train motion to show layouts,
(the prevailing wisdom tends to suggest that the more trains moving on a show layout, the better
Unfortunately, most commercially available "dumb timers" are limited to around 90 seconds in either direction before "changing direction". Rob Paisley has an excellent "home brew" circuit which will do the same job, for a lot less than a commercial one, and gives the ability for the modeller to create their own "long time delay", by choice component selection...
Alternatively, you can simply use the reversing switch on your throttle, incurring NO extra time, expense, or controls to mess with!
NB that if you "change direction" on the throttle WHILE the train is IN MOTION on the loop,
it may "hesitate" if the throttle reversing switch doesn't switch smoothly,
but it will not Stop, or change direction!
(Thats what the Bridge Rectifier is there for).
If you do NOT change direction, the train will simply stop on the diode-fed "stop block", and patiently wait until the Mainline track's "polarity"/direction IS set to accept it.
NB that this can be used to advantage, to power a "green/red LED signal indication",
simply swing a Red and green LED back-to-back,
with a 1k resistor in series,
across the mainline track feed.
The loco will appear to stop in response to the RED signal when it trys to exit the loop, if the polarity/reversing switch is set "wrong",
and will carry on past the green signal if the polarity is "Right"
Alternatively, for those who are more "Semaphore Signal" inspired,
(I'm thinking of the UK NG and Heywood fans particularly),
do the same trick with a Circuitron "Tortoise" or MicroMark "SwitchTender" switch motor or similar,
brass wire-in-tube linked thru the benchwork to a model semaphore...
(The average 0 - 12 VDC on our tracks is plenty to give a nice "slow motion" semaphore motion,
and at around 30ma at stall,
shouldn't add any significant load to the throttle!)
Anyway, glad to hear it's a "working solution",
Looking forward to seeing how the layout turns out
BTW, 2' X 4' layout = large?
That can be an entire empire if designed "in kind",
It's not GnXX, but try these examples,
Broughton Vale Tramway
3 X HOn30 scenes in 2' X 4',
giving continuous run AND purposeful operation.
Designed to fit in the 13" X 19" X 9" carry-on airplane baggage size limits,
HOn30 logging, 4 scenes, and a warm and fuzzy carpet covered box
Aim to Improve,
"...Happily hauling hardwood behind Class B Climax 1375,
over the Pig Sty Pier trestle on Longworths Tramway,
somewhere deep in the Aussie Bush..."
PS "OzTrains" John G is being too modest, his "Yallah 2" layout has working examples of multiple "dumb timer" train operation AND handlaid "1 way turnouts",
and fits an entire On30 promo display measuring 8' X 3' in "Ready to Run" mode,
into a mere 2' X 4' for transport!!!!
(and weighs <20 kilos complete!!!)