General Purpose 10 channel relay panel to control various 18vac layout accessories.

Since the AIU internal relays themselves are rated for 5 amps, what accessories are you planning to use that require the 25 amp rating of your external relays?  Driving a relay with another relay is redundant if high amp capacity is not needed. 

Mainlines or sidings where you would want on/off control.  These could go up to 10 amps with most high end transformers or even 20 with a TPC.

CAPPilot posted:

Mainlines or sidings where you would want on/off control.  These could go up to 10 amps with most high end transformers or even 20 with a TPC.

Except the original poster said, "It will be used with a DCS AIU module to control various 18vac layout accessories."  Nothing about controlling track circuits.

Looks like overkill for accessories unless you have the optional welding accessory.

THESE RELAYS ARE AUTOMOBILE TYPES.  THEY ARE VERY RUGGED AND INEXPENSIVE. THEY INCLUDE A PREWIRED SOCKET AS SHOWN. THE RATING FOR THE CONTACTS IS 25 AMPS WHEN SWITCHING EITHER DC OR AC CIRCUITS.  ALL CIRCUITS ON MY LAYOUT HAVE SEPARATE FUSES FOR EACH CIRCUIT.  THESE RELAYS MUST BE USED WITH 12VDC TO ACTIVATE THE COIL.

Just to throw it out there, automotive relays have a couple downsides for layout use.  These never stopped me from using them, but folks should be aware.  First, of course, they are DC, and as such they are likely to chatter if used with AC power typically used for O-gauge systems.  One could add a diode and small capacitor to each relay for AC use, or simply run the relay panel off 12VDC as the OP does.  A second issue is that most 12 volt automotive relays are designed for high current but low voltage.  I've never seen one fail passing 18VAC, but it is beyond what they are designed for.  Automotive relays should defiantly not be used for mains level AC, as that is well beyond the contact ratings on automotive relays.  

All that said, I'm assuming here that the use of 25 Amp relays here is simply because they are inexpensive and come with pig-tail sockets, rather that because 25 amp circuits are needed.  If a 10 Amp limit is enough another solution may be a relay module board such as this one:  

This board gives you 16 relays for under $20, shipped. Aside from the current being lower the board requires 5VDC supply and control.  It wouldn't matter if one is just piggybacking these relays off the ones inside a TIU, but for other uses, these boards also come with opto-isolators for each channel, preventing any damaging spikes from getting back into control electronics used to trigger the relays.  

JGL

IMO the takeaway is the idea that an AIU relay can "relay" control to yet another relay that has additional/better capabilities than the AIU relay.  It does require a source of 12V DC or whatever the external relay requires to operate.  So not just higher switching voltage/current capability, but maybe the external relay has multi-pole capability instead of the AIU's single-pole.  Or, relevant for accessory operation, an external relay with timing capability such as turning OFF after some settable time.  If the AIU is controlling a UCS uncoupling electromagnet, you want to limit the "on" time; so if you mistakenly leave the AIU relay "on", the external relay with timing capability can turn it off.  For example, here's a 10 Amp relay that has such timing capability for $2.39 eBay (free shipping from Asia).

For large layouts, a power hungry accessory may be 50 feet or more from the AIU.  There may be wiring benefits to sending a low current 12V control signal to a distant relay that does the actual multi-ampere power switching.