Just to put things in perspective,  You are working on What # Santa Fe Engine?
It could be  Postwar? 2333, 2343 2383? 2243? MPC? Modern ERA?  What number is on the engine?
 There are so many things that can go wrong  to cause the Horn not to work in any of these.  Your brother might have put a replacement horn setup in the engine. I say this because 9V Is a bit too much voltage for a postwar horn. They did not come with 9V connections. They worked on 1.5 volt D cells. 9V can damage the coil in the horn of the earlier engines. There are 9V replacement horns available for replacement in postwar 2000 series engines. If so, and that's what you have, It may not be using the original horn relay to control it. It my be using another activator that came with the replacement, like a board.  Can't really say without being able to see the installation.  You say If you connect the 9V battery, the horn goes off, then whatever controls the horn is stuck on or is activated and needs to be looked at as to why  it cant be controlled properly.
Sorry I can't be of more help but with the info provided, it's all I can think of.   

All,

This is in follow up to my previous posting on the reversing gear in my Santa Fe diesel.

I am trying to activate the Horn. When I insert a "D" size battery (either up or down) the "whistle control" from the Transformer doesn't activate it.

There is also a 9V battery connector in the battery compartment, that may have been an "add on" from my deceased brother. If I attach a 9V battery to it, the horn blows off!!

Any advice on making this all work would be appreciated!

aaganz

To all,

I miswrote when I referred to the poor performance with a 90 Watt transformer. I looked again this morning under the Christmas Tree, and see that it is a large 175 Watt transformer running the Santa Fe diesel on 027 gauge track. The engine runs fine, although reversing of the A-B-A + 3 passenger cars is pretty slow!

However, the 175 Watt Transformer really powers my Steam engines and Milwaukee diesel with a full set of freight cars very well!!

aaganz

I do not know the age of the Santa Fe diesel, as my (deceased) brother was an avid train collector, and had been acquiring Lionel trains for a long time. The locomotive number is "2343". The side ladder to the cab has 2 rungs. There is a "GM" logo on the side near the aft end of the engine. When I slide over the battery holding plate, there is an imprinted number  "2333-20".
I have attached the train set up as an A-B-A configuration under the Christmas Tree. (Dec 2025). The pix failed to upload! Sorry.
Let me know if a closer photo of the engine itself would assist.
Thank you in advance for any help you can provide!
aaganz

Hello & Welcome to the Forum.
How old is your set? This IS important.
The Santa Fe set has been around for a very long time.
Set number/locomotive number? 
Researched the Lionel Trains Workshop sub forum for an answer?
Yes, that 9 volt battery, should be replaced with correct voltage.
Photo's are always welcome.