Postwar horn only works on 9 volts.

I had similar issues with these horns.  
My experience is that they get stiff from old age and disuse  

Some of them hem I got working y jolting them with 9v as you have   I’ve done it a dozen times or so for 2-3 seconds it not constantly   I take a break after 4 times to make sure the coil does not overheat  

I had all but one return to working with 1.5 V  

make sure you test directly to the horn and not through the relay  so as to have on less item to worry about

I've had good luck converting my PW to 3 volts,I use AA or AAA in a holder,it works fine...

Good idea. Good to know that 3V doesn't kill it.

does it sound the same?

Is it the original horn?  Many replacements that look similar could be higher voltage.   

I use battery replaces and some I had to set to 2-2.5 volts to keep the the horns working well. 

@joe krasko posted:

I've had good luck converting my PW to 3 volts,I use AA or AAA in a holder,it works fine...

That sounds like a possible plan. Where did you attached to hot wire?

@VHubbard posted:

Is it the original horn?  Many replacements that look similar could be higher voltage.   

I use battery replaces and some I had to set to 2-2.5 volts to keep the the horns working well.

I don't know if it's original or not. I bought the engine used. It looks very clean.

Can you give me some more information on the battery replace item you speak of?

@Train Nut posted:

That sounds like a possible plan. Where did you attached to hot wire?

follow the path of the D cell....attach the + wire to the top of the relay...good luck...joe

Consider using a simple circuit that converts track volts to 5VDC.  For any loco that uses AC track voltage, a basic half wave circuit with one diode, a filter cap of 100uf or so at 35V rating, and a LM 7805 regulator will give you a steady 5VDC from your track voltage of about 6VAC on up to full power.  Making it half wave means you don't need a separate ground or DC return wire above chassis or common rail track potential, and the 7805 can be screwed to a metal chassis with no insulation.  Several circuit diagrams are available online. There is a regulator chip that will give you 5 volts DC out from as little as 1.3VAC in, used in MTH locos, but forgot the number.  If 5 VDC is too much, measure the current the device draws at normal battery volts, and use Ohms law to calculate a series dropping resistor value to reach you required volts. Ex.  Your load draws 100MA at 1.5 VDC, you have 5VDC you need to drop it 3.5.  R=E/I    R=3.5/.1 = 35 ohms or so will give you 1.5 VDC at your load with a 5 volt source. Does not need to be exactly 35.   27, 33, and 39 are common values you could use.  Just an idea, and an easy way to add LED lamps to track bumpers.  All parts are available on Amazon, and you can build dozens of them for a few dollars.

Many O gauge sellers have "Battery Eliminators"   Basically a buck convertor that is powered through the relay switch.  They come with instructions.   You need a volt meter to check their output voltage.  Some allow you to adjust the output voltage with a small screw on a varistor on the board.    Not to be confused with the BCR/BCRL 9v type.     

I use them since they don't leak battery acid over time.   Adjusting the voltage up a little usually brings many horns back to life.     

I have used 9v batteries, but they still can leak and I am probably safer at 2v when my granddaughter runs the train.  She likes to hold the horn button until everyone tells her to stop.