Maybe this is on your loco (N scale?):
http://www.spookshow.net/f40ph.html
Here's a TCS install, always a good place to look for locos that are covered:
http://www.tcsdcc.com/Customer_Content/Installation_Pictures/N_Scale/Life_Like/F40PH/LifeLike%20F40PH.html
You will want to know if the motor is isolated from the frame, but presumeably the install above addresses whether any action related to that is needed. And you need to know whether the bulb voltage type is compatible with your decoder voltage output, or whether it's a low voltage bulb that needs a resistor. Plan to get familiar with LEDs in locos, as it's not a whole lot more to replace any bulb with a LED with resistor, and essentially eliminate the issue of future bulb burnout maintenance. It mostly becomes and issue of having the right parts inventory available: 5mm and 3mm LEDs, 1k and a few other resistors. Your inventory will want to have some 30 AWG wire available as well as some small shrink wrap tubing.
EDIT: Just noticed the date...15 months ago! And no replies then??
Paul
Modeling HO with a transition era UP bent
My Layout Photos- http://s1293.photobucket.com/user/ajwarshal/library/
Its pretty straight forward as I did it on a AMTRAK loco I had. Just make sure the loco runs great with DC before you do the modifications. I think the hardest part is finding a place to put the decoder at least it was for me in N scale.
I just got the digitrax dh126d mobile decoder and I want to install it to my Life-Like emd f40ph. the connections looks simple but this my first upgrade. is there anything I should know before I start? I can not find much detailed information on the engine.