Cracks in Concrete Ceiling

 Let me first of all congratulate you and your colleagues for an excellent web site and the information provided. This is the second time that I'm am requesting guidance in this area and very much appreciate your time. I have browsed through the frequently asked questions and have a new problem.

My situation is as follows: I have a poured concrete slab ceiling directly over a transformer vault. There are numerous hairline cracks visible to the eye. Water migrates through these cracks on a rainy day and consequently leak on the transformers and switchgears. The concrete slab doubles as a handicapped walkway entrance (and roof to the transformer vault) to our business building. The transformer room is naturally below grade. The handicapped walkway/roof finish is coated with a non-slip epoxy coating.
This appears to be in good condition and the only waterproof membrane indicated on the drawings. I am somewhat surprised that this passed inspections. Wear and tear on the walkway finish naturally would accelerated the life of the membrane.  My question, what alternatives are available to seal these hair- line cracks?.. Painting the underside of the ceiling?,...
hydraulic injection of a sealant/caulking into the cracks?.. routing out the cracks and installing a rudder strip and caulking? refinishing a epoxy coating, or some type of roofing membrane that can double as a walkway.
It is really nice to hear from you again. Thanks for the compliments.  Check again in a couple of weeks after I switch to a "frames" site with additional content.

This time it sounds like you have a serious problem that must be fixed.  I understand that:
1.  This is a structural slab with two-way reinforcement (or post tensioned?)
2.  There are cracks that appear closed (hairline) but which let water through.
3.  There is a waterproof membrane over the concrete; epoxy based; in good condition.  Does that mean that you cannot see the concrete or cracks from the top?

Questions:
1.  From below, do you see any deposits?  Yellow/white like last time, or rusty?
2.  What kind of traffic are we talking about?  foot, carts, wheels, light, heavy?
3.  How old is that slab?  Is it done curing, cracking, and developing its strength?
4.  Is there a contractor who built it and may be willing to fix it?

Solutions:
1.  The best would have been through original design of two part slab.  After the structural slab is in place you apply roofing membrane and pour another 1.5-2 inch plain slab on top.  The top one serves as wear surface that protects the membrane.  Usually you provide drains at the edge of the slab to collect the water that pass the top and runs over the membrane.  It is more expensive to install but there is no maintenance later.  If you have the money, and the clearance to add 2" of concrete - go with that.  But I doubt it.
2.  Sealing the underside is practically impossible.  There are some products that claim to do it but I hear so many complaints that unless someone guarantees it and shows prior experience I would not trust it.
3.  A readily available solution will be to remove the existing membrane to bare concrete; possibly sand blast the surface; and then apply one of the many industrial type epoxy finishes.  This expensive solution will give you a very strong wearing surface and you don't need to deal with the cracks themselves.  Concrete surface preparation is vital to ensure that the epoxy penetrates the concrete and creates a good bonded surface.
4.  All the options of crack repairs you mention are not perfect and might reopen when the concrete 'works' under thermal  and moisture variations.  They also require maintenance.
5.  Some type of roofing membrane that can double as a walkway - if it is designed for the kind of use you anticipate and is guaranteed.
 
Good luck and let me know what you decide,