I just want to know why everyone keeps going into the tunnel to begin with. You would think there would be a sign or something saying "Hey, trains go through here. It might not be a good idea to use this tunnel if you are a pedestrian."

The tunnel is a metaphor for life. Why do we live to begin with? Its dangerous and full of everything terrible to trap you... or better to test you! This Tunnel we call life, has the train, the certain death. Its you who are to decide wether you look at it optimistic, or pessimistic. I chose to give no fuck about it.

But if life is a train tunnel that means everyone must go the same way, with no way to make your own path.

The tunnel is life, not the way we run through it. A prostitute walks through the tunnel as a prostitute. A bible thumper goes through it as a bible thumper. The rail is the time, you could not turn back, even tho it seems so. Your own path is a different metaphor, but this one fits free choice into the time lapse. You born in the begining, and you die in the end where a train hits you. Train being death's metaphor here means that you misunderstood the metaphor.
Life is in fact a tunnel, and no one can go any other way than death. But they can walk it any way they want.

Here is the problem I have with that metaphor, it assumes you are alone in the tunnel and all you can do is see how far into the tunnel you can go until you get squashed.

That is because its your life. You cannot walk anyone else's life, but your own. You are locked into your own life tunnel, and cannot change to, for example, Jennifer Lopez's tunnel. Its a metaphor for your life and the time that passes in it, and not the space you spend it in.

But you cannot deny that peoples tunnels can merge and separate, were they walk and how they walk effects everyone who their tunnels come in contact with. Life is not a tunnel but a whole subway system, you cannot walk someone else's life FOR them you can walk it with them.

If the tunnel is life, then why are there other tunnels in it? The thing is, everybody runs on the same tunnel. Pete as a chef who cheats on her wife. Her wife as a prostitute, because the man walking beside her, Pete, cheated on her. No matter how we look at it. Its the same life. And life has multiple people in it. And all of them walk the same tunnel. They can die earlier or later. But they cannot have any other escape from life. nor turn backwards and walk to their birth. Once again you misunderstood the metaphor's point being time and life.
Because no matter how you die, its death. The end of the tunnel for you. Your life and her life cannot run into each other and make a branching for the tunnel. They die and live their lives.
Thus, I deny that tunnels can merge and separate. That would mean worlds merged.

That is said, maybe saying the tunnel is yours was a bit of an error on my part. The tunnel is life, and time, in general. And everyone walks the same tunnel.

But if everyone is in the same tunnel then the train comes at the same time for everyone, meaning life ends for everyone at the same time.

Not excatly, life is a wide tunnel so they could avoid it. The sick, old, and thus incapacipated ones are those who are hit by the train.
Of course, a pessimist might not notice the train because he is too busy watching the dark. The optimistic may, or may not realize its a train and jump away. The Realist jumps away the time it flashes out. And that can go on until they grow too old to jump away. Or too sick. In real life, only those die who happen to be old, deadly sick, or suffering a fatal accident. Others arent sick at that time, and are still young. They cannot die also because they arent in the accident. That means, the train hits selectively based on who stands on the rail, without attention, or being sick, old or stuck in the rail.
They may still manage to be saved in the last moment. They may still die. Sometimes the train misses everyone, because on those days, no one stays on the rail. And continue their quest beside the rail, until the train leaves and they can stand back.
You can notice here that the rail and its surroundings have their roles. The rail itself is where people stand when everything is normal, until the train comes. The space on its sides are the space given to people to stand away from the train that comes for, example, pete. That is Pete's Train. And even Pete can avoid it as long as he is not stuck.
Those who die didnt make it in this metaphorical tunnel. They were all stuck in the rails due to elderness or sickness, or even worse, just plain leaped into a hole between two planks in the worst time. The others may cry for some while in the tunnel, but they cannot turn back and save him, for he is already dead. (Bible even says "Thou shall not look back" which means you are to leave the past behind, especially what hurts.
Sometimes tho, the train can derail, meaning it will kill some people on the side it falls into, leaving those realizing the sudden change and jumping to the other side of the tunnel alive. Its a complicated thing, because that train is not very long, maybe its even just the locomotive. But sooner or later, it will hit everyone. No matter what. Life ends in death. Avoiding it forever is impossible.
Your life can only begin and end. Nothing else. And you are to enjoy the long long stages inbetween. That's what personal freedom really means.

In short, You can only go forward in the tunnel -time- in a normal pace. There is no such thing as "God's path and Satan's path". Only time exists. And everyone goes as something else on that path. And that something is their own choice.