You wire the AC voltmerers {Vac] across-the-line, one terminal connected to the transformer Hot output conductor[from "A", or "D" post } and the other conductor to the transformer Common "U" post or outside rail circuit.
The Amp meter [Aac] is wired "inline" of the hot wire circuit. On most [edit: see Bob Nelson's opine] meters the feed wire from the transformer "A" or "D" post is connected to the (+) terminal post of the Aac meter and the (-) meter terminal post connects to the conductor out to the center rail. This +/-connection is only important to properly orient the meter pointer.
Google the Shurite Meters which are affordable Toy Train AC analog "moving magnet" meters. They will run about $25 each or less. Higher quality "Iron Vane" meters run about $85-115 each and are unecessary for Toy Train circuit monitoring. The moving magnet meters are about 2-2-1/2% accurate.
When reviewing the catalogs--Mouser, ALLElectronics, Newark Electronics, Shurite,etc, be sure that you are looking at AC Meters not DC which are very prevalent in the llistings. Generally you want about an 0-15 amp range and an 0-25 volt range in your Toy Train power district meters, but other ranges are available.
I was importing DER 670 meters a few years ago at a USA landed/customs cost of $28-$32 a pair [one Aac, one Vac]but today the air freight cost in the Pacific Rim has raised the single meter cost to about $25 each.
You need a pair of meters for each separate layout power district and you want to connect them at or near the transformer terminals to read the totalload upstream. The amp meter requires heavy gauge wire [#12 or #14]because it carries the entire power district load. The Voltmeter can have light wire [#18] because it simply is reading across the line.
Most all of the meters require a 2" round hole in the masonite,aluminuim.etc that you mount on; for the "barrel" of the 2-1/4" x 2-3/4"' face or dial[typical] and the acrylic face has 4 bolt [machine screw] holes.
