PowerShell, 116 bytes

param($n)(,(" "+"____"*($x=$n/2-replace'\..*')))[!($n-1)] " /\ "*$x+(" /"," /\")[$n%2] "/__\"*$x+("/","/__\")[$n%2] 

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Not allowing an empty line for n = 1 ate up like 14 bytes. This solution is pretty brain dead with a lot of repeated code. Banker's Rounding is still the actual devil.

PowerShell, 116 95 bytes

^{Big thanks to Mazzy and ASCII-Only for saving 21 bytes}

param($n)@(" "+"_"*4*($x=$n-shr1))[$n-eq1] " /"+"\ /"*$x+"\"*($a=$n%2) "/"+"__\/"*$x+"__\"*$a 

Try it online!

Not allowing an empty line for n = 1 ate up like 14 10 bytes. This solution is pretty brain dead now much more clever with a minimal amount of repeated code. Banker's Rounding is still the actual devil.