Cube should not slip
I have a moving platform, with a cube placable on it. It is absolutely important that the cube keeps its position (so you can jump on it, bounce off, avoid the turrets at the platforms level, jump again and so on). Now every time the tracktrain stops and starts in the opposing direction the cube slips a bit. If I do DisableMotion the cube gets stuck above the platform. Another way, I thought, is kind of snapping the cube on the platform with a dummy, but that is a "no-way-else-soloution". Parenting the cube to the platforms tracktrain does unfortunately not override the "disableMotion". Is there any way to have no slippy cube, but you can move it free?
Usually, in order to achieve that, you have to place a thin func_door brush on top of the plattform. That trick will avoid your cube from slipping away... Texture it with the invisible texture.
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Another way would be to design the platform with a shallow, rectangular hole in it. That way, the cube will not silde, and the player will understand what to do with the platform. The solution may become obvious by doing this, though.
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The door trick did not work.
I use the platform model from Chapter 2 Chamber 4. Have a look at the vmf. The entity is name "ch2_ch1_platform_train". Goal is to have the blue painted cube placeable on the platform and also off the platform, but no movement while on the platform, seen relative to the platform 
NOTE: If you playtest the map, be sure that you hit "ch2_init_button". As the name says it initializes the chamber. By the way, this button with the arrow starts the platform 
But if it's blue-painted, doesn't it just launch in random directions?
Nope, its a jumpenturret . Due to a lack of programming, prop_monster_box does not have the crazy jump behaviour, its just kind of cube-shaped platform you can bounce off.
I've looked into your map. My func_door has only the flag "Toggle" checked (yours, "Starts locked"). Also, I use to set to YES the "Force closed" property... I cannot compile your map cause of missing instances, so try that and let me know.
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