The cache is not at the given co-ordinates.
There are many interesting relics you may come across in Mottram Yard if you venture off the main paths and look around. I had to archive the previous 'side of the sidings' cache and rather than simply stick out another box, I have livened it up, but don't worry, it's nothing very hard. The above picture shows the platform you need to find. You need to use your GPS to navigate to the given co-ordinates, which are within the Mottram Yard area. When you stand at the given co-ordinates you will be at a junction with various paths and directions. I'm not saying where you need to go, but you should be able to see the platform from a path (as most of the paths follow old trackbeds) and the platform is within 0.2 of a mile from the start point. You shouldn't need to go crashing through the undergrowth and definitely not cross any fences. The aim is purely to get you to look around the area in more detail, however, it shouldn't be massively difficult to locate.
To locate the box - When you find the platform, you will see there was originally a piece of wood that overhung the platform edge for it's full length. You will note much of it is missing, but some remains. You need to find where the overhang now stops - the end facing me when I took the photo (see spoiler arrow). You will find the cache at that point. Obviously, you do not need to cause any damage to the platform in order to retrieve the cache. Please fully re-hide to avoid it getting muggled by people exploring the platform.
This huge cutting was once Mottram Yard, and 20 sidings once filled the whole area between Broadbottom and Dinting Viaducts. In the industrial times, these sidings were constantly busy day and night as goods were brought to the sidings and re-organised into huge freight trains to be hauled over the Woodhead line, originally by the powerful steam locomotives and later the heavy duty early electric locos.
Back in steam days you would have found a 300 foot deep well not far away, used to fill a 50,000 galleon tank. With between 40 and 50 steam trains needing to take water here every day this water would soon evaporate, literally.
The original viaducts were not massively strong, built for much lighter trains. When steam was at it's peak the line was so busy the owners seriously considered building a second viaduct next to the existing one at Dinting to take the heaver trains. This would also expand capacity and allow the existing bridge to serve the Glossop line and Dinting yard. However, eventually the existing bridge was altered to include the additional brick supports we see today.
When steam was replaced this important line was quickly electrified in 1954. Mottram Yard still played an important part as diesel engines would haul much of the goods here, where the trains would be switched to powerful electric locomotives to pull over the Woodhead line.