Very sensible approach on the face of it, provided you get the initial design right. By right I mean that when and if it's time to expand further then it can be done quickly and with the minimum of disruption. The last thing you want is to lose half the stadium during the season because you're expanding.
Rail links are notoriously expensive and can be a bit of a white elephant. How many people do you think will be able to pack onto a single train to get back home. In London they close tube stations if there's a big event at say Hyde Park. To prevent the crush and overloading of trains. There's also been plenty of recent history of recent inner city train links that have bombed one way or another.
Finally there's the huge question of who owns and maintains this infrastructure when it's all been built. Everything but the stadium and parking. The Local Authority is the obvious choice BUT that brings in another wobbly or two. Firstly it has to be built to meet the necessary standards (easy enough) AND whatever stipulations they place on it. Secondly they will want a commuted sum to take ownership and manage it all for the rest of it's life. That's not cheap.
So basically you have to pay for the design, planning approval, procurement and then building it. Then to top it all off you'll need to pay up front for someone to look after it.
I'm not trying to knock the idea. I'm all for a new stadium, I think we seriously need to move on from Anfield. As great as it has been the whole club needs to move forward. I'm just trying to point out how difficult it can be. My work gets me involved in a lot of this kind of stuff and I see first hand the trouble, costs and problems that are encountered.