East Side Access is basically going to create a brand new 8-track stub terminal (tracks 201-204 and 301-304)
deep under Grand Central Terminal. When all is said and done there will be no track connections between the two railroads at Grand Central.
Right now they're basically excavating this
giant cavern which will hold the eight tracks, 4 island platforms and passenger concourse.
The LIRR concourse would be separate from the MNR one, and you'd get from the MNR portion of GCT and you'd get to the LIRR concourse and tracks via these large
escalator banks down to the lower-lower level.
Once your there there will be this concourse that stretches the length of the lower-lower level. There will then be staircases to the 4 island platforms, 2 directly above the concourse, two directly below, (two on the left wall, two on the right wall). It would eventually look like
this,
this,
this,
this,
not this, and
this.
And because diagrams are always fun,
I submit this one into evidence of a cross section of Grand Central terminal come 2019.
The LIRR forum has an extensive
East Side Access Project Discussion ongoing. Almost everything's been covered in that thread all the way down to speculations on which trains are going which places come 2019. Aditonally, the
MTA Capital Construction East Side Access Project Site and MTA Capital Program dashboard are good tools for all things press releases, diagrams, etc.