Nice idea, but... what happens when these things start leaking toxic fluids into the already contaminated water system?
How do you perform maintenance on these pods? Are lubricants required? Do the electrified rails effect the water quality, or city services' ability to work on the water pipes? Electrolytic action could be a problem too. I can hear the jokes when a package arrives, "Here comes the poop.", "We're about to get crapped on.", "Why do these packages smell funny?" New delivery service, DDS (Duty Delivery Service). But UPS is in good shape, they already got brown (brown underground).
While on this subject, why do so many people prefer to buy bottled water rather than clean up the city water supply? Where I live I've watched the water system get steadily more polluted over 10 years. When I moved here the water tasted good. Now its tastes like a swimming pool with too much chlorine in it. You can smell a glass of water 10 feet away. And it tastes like a$$. The water reports show all the pollutants going into the water. People.