My house is located some 38,000 blocks away from the house of another player on my SMP world. We are planning on creating a railway between our houses, and I was wondering if I could send a storage minecraft with items down the railway and if it would reach his house without me accompanying it?
Sorry there isnt a better answer but as soon as it goes out of an unloaded chunk, it stops getting updated and will not move any longer. There are mods out there that can keep chunks active for a server so you might want to look into those, but not by default.
UPDATE: In the latest version of Minecraft there are devices called Hoppers. You can use these to keep chunks loaded and in theory can keep your track loaded so the cart would make it all the way. However Hoppers are NOT friendly to the processing speed of Minecraft. So this may have very adverse effects. However, if you want to try it out and see how it goes, you can hop over to this question to see how they are made: Can I use a hopper loop as a chunk loader?
There is no way to for a mechanism to pass through unloaded chunks or cause chunks to be loaded, but here's something you could try to make sure the minecarts eventually reach their destinations. I'm not certain whether it would work.
Arrange so that just before each chunk boundary, there is a downward-sloping powered rail. Then, as soon as the next chunk is loaded and the cart is able to move, it will move down the slope and into the loaded chunk.
If just being on a downslope isn't enough to prod the cart to move (unlikely), you could add a clock nearby to toggle the powered rail periodically.
With enough piston activity in a chunk it stays loaded. If you make a flying machine between 2 chunks (make sure part of it passes into the chunk next to it or an auto save will unload the chunks and stop the machine) all the chunks that it passes through will be loaded as long as its there. You can use this method to transport items if you put a minecart chest on it, however a server restart will still unload the chunks. This is solved by a sheep. Sheep still eat grass even if they are in a minecart. Put some grass with a named sheep in a minecart on top as well as a light so the grass grows back. When the sheep eats the grass, it will trigger an observer which can start the pistons back up. The only problem that I still need to solve is how to load the chunk with the sheep so it can eat the grass