Marty, Yes, I have installed several, all in steam. And all in the tenders. How difficult it is depends on your soldering skills. Other than soldering, there is nothing particularly difficult. Finding or making room can be a problem depending on the model. My most recent installation was in Bachmann's Vanderbilt tender, a tight fit, but it turned out to be an easy job. I could post some photos of the install if you would find it helpful. Most tenders have sufficient room for the decoder and speaker, with the speaker going under the coal bunker and facing up. I used the relatively new N scale decoder for the Vanderbilt tender and it worked well. My other tenders fit the regular decoders with no problem. A diesel installation would require more effort to fit, I haven't tried one yet. There are photos of diesel installations at
www.ttx-dcc.com
When installing the decoder in the tender, several wires need to be run to the loco from the tender. I have standardized on five wires. Two for the motor, two for the headlight, and one for track power. Some of the newer steam locos have pickup from both rails on the loco, these use 6 wires. For my 5 wire installs, I use 5 pin connectors, with the female fastened in the tender, and the male at the end of a cable from the loco, allowing me to unplug and seperate the units. The 5 pins are in a row, and the ones I use are not indexed, so I use the outer two for the motor, next two for the headlight, and center for track power from loco. This way if I plug in reversed, I won't accidently connect track power to the decoder output, probably a bad thing.
To sum up, if your soldering skills are good, you should have no problem. If you need help on soldering, ask and I am sure several people will respond.
Good luck.
Gary