The emergency track access ramp shown in pic 1 is accessed via a substantial gate not shown in the pic. The roadway from the access gate leads to a lay down area which is just to the left of the barrier at the top of the ramp. Track access would only be made during possessions and I suspect that the reason there isn't a barrier at the bottom is to allow speedy access in the event of an emergency.
The main access gate will have a sign detailing where you are, lines on site, mileage, line speed and direction of travel. It should also have relevant info about the OLE^▸ (Overhead Line Equipment, more often "OHLE").
The cable route you see was probably original so rather than cut and replace the cables through a ducted route they were lowered and protected. It's been a long time since I've been there so going from memory.
The original access for the tunnel portal was via a set of steps, you knew you'd climbed up and down those. I remember replacing  all the tunnel telephones and their associated cables, I walked the tunnel from end to end 3 times in one shift keeping an eye on the lads doing the work.
I can also remember going out with a couple of scientists who wanted to look at the rare creatures resident in the tunnel water pools, these guys were super enthusiastic but not so when the first pool we came to had a perfect size 10 boot print in the middle of it.

The Bristol East works is a track remodelling project and nothing to do with resignalling, that’s already done and dusted.

Unterminated wires can be taped up but only for the short term, can't remember the exact timescales but think it's 24hrs. After that time the wires must be bomb tailed or removed. The idea being that if you didn't have any crimps but did have a toll of tape then you could make the wires safe. I can't quite see in the pic if the wires are disconnected or not but green tape means redundant anyway so I'm wondering if they're just marked up for removal once the mods are completed.

https://www.youtube.com/watch?v=RSCz-35M9hA

Well I hope I've managed to post this link properly and if so it shows what can happen if you don't follow the rules properly.
The American system of dealing with buried services differs from ours in that the utility owner has to attend site and mark up where their assets are. In the case shown it was known there was a gas line in the area but the owner didn't know exactly where it was. The contractor was told to stand down until further investigations were carried out but as the job was nearly finished and behind schedule the contractor decided to carry on with tragic consequences. Scary stuff.

Well this is my first post and it's about a subject I know a little about I thought I'd join in and try to dispel some of the myths surrounding the whole buried cables issue or indeed non issue.
First off, all the available drawings for the Western Route were collated and scanned some years ago, as the odd ones remaining become available then they to are subjected to the same process.
The rules surrounding buried services are laid down in H&S^▸ (Health and Safety) law and all industries incl the railway have to comply. The rules are quite simple really and set out a set of steps which if followed should reduce the risk of damage to any services within the work area.
I'm not going to go into the full details but let's look at a scenario where someone wants to install some equipment which involves excavation.
             RULE 1. UNLESS YOU KNOW OTHERWISE NEVER ASSUME THERE IS NOTHING THERE. ALWAYS FOLLOW RULE 1

Step 1. Obtain where possible drawings showing the services in the area. None available or not up to date go to Step 2
Step 2. Ask the asset owners if they know of any services in the area which may not be on the drawings. If unknown then go to Step 3.
Step 3. Carry out trial holes and fully CAT Scan the area. Please note that not all services show up with a CAT Scanner.
If services/obstructions are found then note and report the issue. In addition use that info to update the records.
Step 4. If the area is clear then carry out your work.

It should be noted that this process is for buried services which on the railway can include lots of things; drainage, cables, culverts, old foundations, old sleepers, rail never recovered, the original foundations for Swindon station etc etc. You can never assume anything when it comes to buried services and even if you had the drawings you MUST always do further investigation work.
There is actually 1 step prior to step 1 above which may be useful: Find an old git with the local knowledge who knows cables and ask him what's there.
I also see above a reference to the Filton Bank Project. All the redundant cables, where possible, have been recovered and any left in the ground are known about and plotted. The plastic ducts you see are temporary protection until the new trough routes are installed.

Hope I haven't waffled on too long for my first post and I'm happy to answer any generic questions about cables on the Railway.