On diagonal tracks, the signal dragging causes some problems.
Case 1: diagonal track pointing towards the player. Drag signals with distance 2 (default) towards yourself: result OK. Now repeat, starting and ending on the same tiles as in the first action (or end dragging on another tile with a signal placed in the step before): the last signal becomes invisible and points into the opposite direction. Quite an obstacle for trains and then for the player, because it remains invisible unless save+reload ist used.
Case 2: diagonal track parallel to the lower screen edge (assuming that you have a rectangular screen not standing on a corner :D ), drag twice (EDIT: from left to right) as in case 1. Result: the last signal will become bidirectional. Harmless and obvious, but still not correct, possibly with the same reason as in case 1.
EDIT: dragging into the opposite direction in either case will swap the results.
On straight tracks, I cannot see this problem.
Found with Pak128.Britain r1338, reproduced with Pak64 r1336.
EDIT: maybe caused by the fix for: http://forum.simutrans.com/index.php?topic=12892.0 (http://forum.simutrans.com/index.php?topic=12892.0)
Should be fixed in r6967. This bug is completely unrelated to the other bug report. In fact, this bug has been around since the advent of dragging for signals (~ 3 years ago, r3829).
Oh, thanks.
I started using the dragging of signals quite late, changing my building habits slowly. Still funny that I stumbled over this so late.
Case 1 appears to be solved. Case 2 still occurs: the last signal will become bidirectional on diagonal track if any dragging action ends on a tile where a signal will be placed. The player may need to fix it manually.
Please try 6983. Both cases were particular instances of one and the same bug.