GG5: 〔灯火管制用の〕 (窓の) a blackout curtain; (灯火のまわりの) a shade; 〔テレビカメラの〕 a flag.
中辞典: a blackout curtain (灯火管制用の); a shade (灯火のまわりの).

The 灯火管制 context may be a bit dated, but I really can't see grounds for "esp. in vehicles". The "灯火のまわりの" certainly looks like a lampshade.

@@ -13 +13 @@
-<gloss>blackout curtain (esp. in vehicles)</gloss>
+<gloss>blackout curtain</gloss>
@@ -14,0 +15 @@
+<gloss>lampshade</gloss>

遮光カーテン	60993	95.3%
遮光幕	2989	4.7%

https://ja.wikipedia.org/wiki/遮光幕
遮光幕（しゃこうまく、英語: Shading curtain）は、光を遮るための幕（カーテン）を指す通称である。
Section 1: 鉄道車両
Section 2: バス車両
Section 3: 旅客船の最前部窓

I guess this is on customer concerns about train drivers using them while driving...
https://www.kotsu.metro.tokyo.jp/enq/customer/pdf/customer-december_r02.pdf
（回答）
都営地下鉄での遮光幕（カーテン）の取扱いについては、列車運転時における駅の進入時に光の反射が強く感じられる場合などには運転の支障となるため...

"lampshade" feels improbable, and I can't find much to support it. "shade" should do the job if necessary.  Is there any source for this outside of gg5? (I see it was added deliberately). 

I do see this fragment in the first google book result:
【遮光幕】(名) (燈光管制的)遮光幕。◇ ~をかける/掛上遮光幕。
But I don't think that makes this a "lampshade" (conical thing on a lamp). And this use would be [obs] or [arch] or something...

The most common use is 1 shade separating the driver's compartment of a train from the passenger compartment.

Image search is all trains. 

yourei results are dominated by vehicle equipment layout.

I put what I thought was the minimum gloss to say "use the xref when curtain shopping". This is [rare] (at least today) for "bedroom curtains", but it appears to have an active use in vehicles.

@@ -12,2 +12,3 @@
-<gloss>blackout curtain</gloss>
-<gloss>lampshade</gloss>
+<xref type="see" seq="2266650">遮光カーテン</xref>
+<gloss>blackout curtain (esp. in vehicles)</gloss>
+<gloss>light-blocking curtain</gloss>

gg5

Looks like a typo. Thanks.

I can't find a single source suggesting that this word is read as しゃっこうまく.

@@ -8 +8 @@
-<reb>しゃっこうまく</reb>
+<reb>しゃこうまく</reb>

GG5, etc.

@@ -11,0 +12,2 @@
+<gloss>blackout curtain</gloss>
+<gloss>lampshade</gloss>
@@ -13 +14,0 @@
-<gloss>blackout curtain</gloss>