Caution - Always test all transformers before use!!

Out of curiosity, was the power supply listed as "regulated" and did you test it with a load of any sort?  Many  inexpensive power supplies are unregulated and do run high when tested without a load but would be fine and read near the listed volts when used in most actual applications.  Springing for the more expensive regulated power supplies is often the better choice.

+1 for what Andy says. Not uncommon for a nominal 18V wall wart to read 25V with no load. The 18V value is at rated current draw.

Pete

+2 Probably nothing wrong with it. Elcheapo conventional DC power packs for HO trains will measure similar until you put a proper motor load on them.

My $460 MTH Z4000's measure at 25 to 26 volts with no load under load they read 21 to 22. Its not jsut the cheap DC wall warts that read high without load its the expensive AC transformers do as well

And to add to confusion, some cheapies are just transformer and single half wave diode or full wave diode bridge and NO filter capacitor, theory being better to filter close to electronics and on board regulator.  Kinda rare but they're out there just to save a few pennies.  So if you add a capacitor, the DC might go up even more.  More iron in transformer gives better output regulation but then $$ and weight (Rat Shacks were notorious for getting away with the skimpiest transformer, largest amperage rating and were great hand warmers).

Andy, it was Not a regulated power supply.  I did not meter it under load.  My meter read 25VDC instead of 18VDC.  That seems to be an excessive voltage difference for 500mA output.

It did fry the rechargeable appliance when I tried it as a replacement for the old 18VDC wall wart.

Hello my friends, My favorite AC transformer is the Marx #1669 100 watt transformer. I find them at train shows for 5 to10 dollars to me that is a good deal for a great transformer and they always seem to work I love them. I also like the red button reset on this transformer it works great too. So if want a great transformer look at Marx #1669 it great for small layouts you cannot go wrong with this transformer I think Thanks longbow57ca.