If you ever thought a voltmeter might be an interesting thing to add to you controls, the font of all stuff, epay, has several 0 to 20 VAC meters listed. Options include prices about $5 for the meter and $5 to ship from China and a month or two wait, to a US distributor selling the same item for about $13 and free 4 day shipping. Item #401288028806. So, plunk down big bucks for the ZW-L to get the metering, or roll your own. Amazon is a great place to find things like plastic or aluminum project boxes too, and yes, they also have low volts AC meters, but the lowest range I could find was 0 to 30. The normal train running voltage would have the needle down on the low end most of the time. Another thing to look at on Amazon are the LED kits. Hundreds of 5 and 3MM leds in all colors, in one plastic box, some come with a 1 for 1 number of dropping resistors, and are the raw material you need to put ditch, classification, and number board lights on the older stuff, or make block signals or traffic signals, all for an expense in the $10 range.
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AAMOF, I'm planning on that addition myself, at times it would be useful to see which of the four circuits has power at a glance.