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.
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.