Is this currently the same as the original version besides updating it to the current factorio version or is there content that was left on the to do list of the original?
also what are the plans on updating the mod, are you gonna add in the features the original author wanted or make your own thing out of it with the things you want from it?

At this point it is just updated to work with 1.1. From what I can tell so far the original functionality is working. I have not committed to implementing the list of features that the original author had planned as the game has changed a lot from that time. If there are features from that list that still make sense and there is a want for it I can work on putting those features in.

Based on the list from the old mod:

Current Features:
Working - Multiple underground levels
Working - Stairs
Working - Belt-Elevators (speed equivalent to the three base belt types)
Working - Power transfer (one-way per pair, for now).
Working - Research to unlock deeper levels
Not working - Subways, special faster trains only usable underground.
Not tested - Remoting call for other mods to white-list entities. (see change log for v 0.3.3 for details)
Not Tested - Shortcut to flip rolling-stock (default: CONTROL + F).
Not Tested - Ground Penetrating Radar research, allows radars to scan underground levels.
Not working - HUD to help when placing entities that connect to other layers (like Stairs, for example.)
Working - Configurable blacklist and whitelists to customize what entities are allowed underground.

Features coming soon(ish?):
Planned - Fluid transfer
Planned - Signal transfer

Features far away:
Is this needed? - Underground mining
Needed - But I suck with graphics - Many more graphics

thanks