I'm a little confused by the On A Rail achievement in minecraft. Do I need to go to a point 1km manhattan/euclidean distance from where I started. Or If I went down 500m of track went back 500m of different track and ended up 2-3 meters from where I started would that count?
No, you have to travel 1000m (=1000 blocks) from the point where you started in one direction. The rails can go up and down but they have to go a 1000 blocks in one direction.
What I mean is that, you can divert from the path, but you have to have travelled 1000 blocks in one direction.
Let's say there are directions W,X,Y,Z.
W = Forward
X = Backward
Y = Right
Z = Left
Now, if you want to go 1000m in W direction, you can divert and go a bit Y and Z, but they will not be counted, similarly, X will also not be counted. You need to travel 1000 blocks only in W direction.
It does not have to be in a single direction as others have pointed out but you have to end up over 1000 blocks or meters as the crow flies from your starting point.
For example: if you travel 820 blocks west and 550 blocks south this will not work since your end location is 988 blocks from where you began. If you traveled 20 blocks further west you would get the achievement since you are now 1004 blocks from your starting point.
You need to build a huge track in a straight line that goes on for about 1000 blocks. The track can't have any turns or bends and so it has to be a straight line in any direction. You must then ride the track and don't get out until the end. sometimes you need to make it a bit longer than 1000 blocks (i done 1129 blocks long). You are unable to earn achievements in a world that has previously been saved in creative mode.