Originally Posted by: Mr. Ron 
I'm not sure if I had asked this question before or not, but here goes. I watch model railroad videos, especially those of European railroads because they use catenary more than we here in the U.S. do. I see layouts with extensive catenary arrays, but they are running their electrics with the pantographs down, meaning they are not using the cat wire for power pickup. Is there a reason for this? ...
Hello Ronald and a Merry Christmas to you,
Yes this has been discussed before, but don’t be discouraged from asking questions.
The answer is not at all specific and indeed extraordinarily eclectic.
Some folk on our group are passionate vintage Märklineers and model in analogue with the original bent-to-the curves Märklin catenary. I understand, respect and appreciate their pursuit of the original vintage and admire it. They can run their analogue locos powered from the track (pukos) or from the overhead (where they have the added joy of seeing the electric sparks from the pantograph-running wire contacts and also of inhaling the wonderful ozone) thus doubling the number of (electric pantograph) locos immediately controllable on a particular track power section.
In the digital world most power their locos (including the electrics) from the track. This may be because early in the digital era Märklin declared that the digital signal was more reliable from the track than the overhead wire. However, our much-esteemed member from Perth, Western Australia,
river6109, has conclusively shown on his very large layout that you can run many digital locos off the overhead wire without any problems.
Some folk run their digital electrics off the track because they have a mode of scenery wherbye they have all the masts and equipment etc to display the overhead catenary – all except the actual catenary wire

. From what I have perceived of their interesting posts, this seems because they feel that even the thinnest model catenary wires currently made (e.g. by Sommerfeldt) are still much thicker in relative perspective ("scale") than the prototype. Some of these folk run their electric locos without the overhead wire but with their pantographs up. We should note that there are a number of extremely large commercial layouts in Europe that do the same – run their electrics with some pantograph elevation but with no actual running wire to the catenary, whilst having all the associated overhead-power paraphernalia.
Another group of folk have the full catenary wire but run their electric locos with the pantographs down or not fully raised (i.e. just short in height of contacting the running wire). My understanding is that they do this to avoid either wear of the pantograph, or of greater concern, prangs to raised pantographs exposed to unintentional obstruction and other hazards.
My opinion is but a drop in the bucket of our groups’ overall wealth of experience; I have full catenary (Sommerfeldt) and a digital layout but tend to run my electic locos powered from the track pukos with pantographs down to avoid the occasional (but expensive) prang

– but fully up for forum photos and video, however !

.
This is because I have only one electric loco that remains analogue and not converted to digital (a beautiful old 3015) because I felt it sacrilegious to convert it to digital - so I run this loco as analogue using the overhead wire on my digital layout with a common "earth/ground/negative" via the outer rails.
Just my opinion, as I noted there is a wide variety of point of view on this wonderful group forum.
PJ