Various improvements and bugfixes

Clear driver on leave using the detach callback (0.5.x and later)
Fix upright_sprite attachment position
Fallback to the regular node sound when metal does not exist
Set animation to "stand" on attach

cart_entity.lua

@@ -54,6 +54,12 @@ function cart_entity:on_rightclick(clicker)
 	elseif not self.driver then
 		self.driver = player_name
 		boost_cart:manage_attachment(clicker, self.object)
+
+		if default.player_set_animation then
+			-- player_api(/default) does not update the animation
+			-- when the player is attached, reset to default animation
+			default.player_set_animation(clicker, "stand")
+		end
 	end
 end
 
@@ -81,6 +87,13 @@ function cart_entity:get_staticdata()
 	})
 end
 
+-- 0.5.x and later: When the driver leaves
+function cart_entity:on_detach_child(child)
+	if child and child:get_player_name() == self.driver then
+		self.driver = nil
+	end
+end
+
 function cart_entity:on_punch(puncher, time_from_last_punch, tool_capabilities, direction)
 	local pos = self.object:get_pos()
 	local vel = self.object:get_velocity()

functions.lua

@@ -19,6 +19,9 @@ function boost_cart:manage_attachment(player, obj)
 
 	if status then
 		local y_pos = self.old_player_model and 6 or -4
+		if player:get_properties().visual == "upright_sprite" then
+			y_pos = -4
+		end
 		player:set_attach(obj, "", {x=0, y=y_pos, z=0}, {x=0, y=0, z=0})
 		player:set_eye_offset({x=0, y=-4, z=0},{x=0, y=-4, z=0})
 	else
@@ -208,6 +211,9 @@ function boost_cart:boost_rail(pos, amount)
 end
 
 function boost_cart:register_rail(name, def_overwrite)
+	local sound_func = default.node_sound_metal_defaults
+		or default.node_sound_defaults
+
 	local def = {
 		drawtype = "raillike",
 		paramtype = "light",
@@ -218,7 +224,7 @@ function boost_cart:register_rail(name, def_overwrite)
 			type = "fixed",
 			fixed = {-1/2, -1/2, -1/2, 1/2, -1/2+1/16, 1/2},
 		},
-		sounds = default.node_sound_metal_defaults()
+		sounds = sound_func()
 	}
 	for k, v in pairs(def_overwrite) do
 		def[k] = v

init.lua

@@ -25,7 +25,7 @@ if not default.player_attached then
 	default.player_attached = {}
 end
 
-minetest.after(0.5, function()
+minetest.after(0, function()
 	boost_cart.old_player_model = not minetest.global_exists("player_api")
 end)