Inexpensive relay supplier

24 volt 10 amp relays for $4.

www.allelectronics.com/make-a-...-10-AMP-RELAY/1.html

I use these on my layout:

http://www.wolfautomation.com/...nctional-devices-inc

They are 3 cents more though, but the coils will run on AC on anything from about 5 volts to 24 volts with no chattering (your mileage may vary). They come with leads about 4" long that are pre- tinned, have two mounting holes, and an LED to indicate when the coil is energized. I added a terminal block to make for easier wiring, and was able to put 6 of them on a board. I wrote on the board what each wire was for. Here's what it looked like  (picture is from when I was dismantling my last layout)

http://www.wolfautomation.com/...nctional-devices-inc

These relays are made to be used in HVAC systems and are or were made in the US. I bought mine from a local HVAC supply house and paid a bit more than the $10.03 that the online supplier I referred to above.

J White

Nice! That looks like a very neat installation, and the wires coming out make it easy. The reason that we recommend DC relays is so that more sophisticated circuits can be built using diode logic.  Also, if rail detection signal circuits have a habit of chattering due to dirty track or loose (or even the occasional plastic) wheels, a DC circuit is preferable because it can be made slow-release by using an electrolytic cap.

These are nice:

Elk Relays

$7.25 each. ELK is a well-respected home security manufacturer.

I seems that there are lots of relays out there in that price range.  Those are DC and also used in the alarm industry. They have one set of Form C contacts, compared to the Altronix ones I mentioned that have two sets of Form C.  That additional set of contacts is useful when you need to set a condition with the front of a train, continue the condition regardless of train length, and then retire the condition when the last car passes.

2 sets of Form C on one relay also easily allows polarity reversals, if that is needed for two-rail DC running with return loops.  The two sets are also useful when you need to control a variable voltage track circuit and a separate signal or block occupancy pilot lamp with constant voltage.

Yes, DPDT relays definitely are sometimes called for.

Truth be known, the workhorse relays on my layout are standard automotive style relays. They are readily available on Ebay for not much more than $2 each, including sockets!  E.g.:

Automotive Relays

These are SPDT.  DPDT in the same form factor are also available, although harder to find on Ebay. When I need one, I usually pick it up at an auto parts store.

Not the quietest, but robust and easy to mount.  You can't beat the price.

These are DC relays, of course, but I do all of my logic in DC.  N.B.: it is quite possible to share a DC ground with an AC "common", so you can still use all the normal insulated rail tricks, if you are into that.

Another consideration is the control current to switch the relays.  Search eBay for Arduino relay module and you might find something like this:

For just over $2 you get two 10 Amp SPDT relays with free shipping from Asia.  Since these are meant to be driven from micro chips that generally don't have a lot of output current, these modules have buffers (optically isolated) with a separate DC input supply.  If you want conventional 2 wire control you just tie the DC supply input to the control input. These also come 1,4, or 8 per board, 5V or 12V control, LED indication, etc.. 

I've purchased some of the Arduino relay boards, some are dirt cheap!   Also, not everyone needs a 10A or 20A relay, so many times a smaller relay will do the job.

GRJ, you'd probably know this.  Have you run across a train application where driving two independent SPDT relays would NOT emulate a single DPDT?  Ignoring the potential inefficiency of powering 2 vs. 1 coil (albeit with different armature weights), I'm thinking of potential variations in switching times or situations where one of the two SPDT's fails potentially damaging something.

Well, if they don't respond at the same rate, it could be an issue.  A DPDT relay will always break before make, but two SPDT relays would not necessarily do so.  I haven't actually had an occasion to try to use two relays that would conflict, but it is possible it could be a problem.  I doubt I'd even consider doing it for the reason that I'd always want to be assured of the relay operational characteristics.

I used to sell this plug in set up for $10 to people who wanted them for block detection. $8 for the relay and socket alone. This was my cost. Still have a few left for people who want them. I have hundreds of various relays in my parts bins.

Dale H

I have a 24VDC system. I use 2, 12 volt ones in series sometimes. There is no conflict or problems.

This board uses series 12 volt relays to get 4PDT contact setups. Single relays have a resistor in series to protect the 12V coil from 24VDC.

 The NAIS SP-2-P relays are DPDT with 15 amp contacts and 480 ohm coils. They are the best relays I have ever used in model Railroading. I use over a hundred of them on my layout. Click on photo to enlarge

All makes sense.  I was trying to think through specific train applications such as polarity reversing.  It's just the price for these dual SPDT 10A modules is unbelievable -  teasing you to use it as a DPDT 10A module with screw terminals and mounting holes for about $2!

I figure the DPDT ones are cheap enough that I can afford to use the right ones.

Dale is undeniably the relay king here!