><> (Fish), 215 183 158 bytes

99+0{:}1=?.~~" "oo::2,:1%-v -1 oooo "____" v!?  )0:/! " /"oa~~.?=1}:{24~/:oo v!?)0:-1o"\"v!?)0:/!-1ooo" /" /v ~o"/"oa/! !\:0)?!;"\__"ooo1-:0)?!;"/"o1- 

><> (Fish), 215 183 156 bytes

99+0{:}1=?.~~" "oo:2,:1%-v -1 oooo "____" v!? )0:/! " /"oa~~.?=1}:{24~/:oo v!?)0:-1o"\"v!?)0:/!-1ooo" /" /v ~o"/"oa/! !\:0)?!;"\__"ooo1-:0)?!;"/"o1- 

><> (Fish), 220 188 bytes

Far from golfed, but it is my first fish program so far >_< The requirement to not have a blank first line for 1 triangle doubled the size of the program.

99+0{:}1=?.~~" "oo:1(?;:2,:1%-v -1oooo"____"v!?  )0:/! .?=1}:{2+aa~/:oo" /"oa~~ -1o"\"v!?)0:/!-1ooo" /"v!?)0: o"/"oa/!  / v~ )?!;"\__"ooo1-:0)?!;"/"o1-!\:0 

Will get around to an explanation soon.

><> (Fish), 215 183 158 bytes

Edit: Notepad++ was giving me 5 extra bytes due to CR, so modified count accordingly

Slightly more golfed, but it is my first fish program so far >_< The requirement to not have a blank first line for 1 triangle doubled the size of the program.

99+0{:}1=?.~~" "oo::2,:1%-v -1 oooo "____" v!? )0:/! " /"oa~~.?=1}:{24~/:oo v!?)0:-1o"\"v!?)0:/!-1ooo" /" /v ~o"/"oa/! !\:0)?!;"\__"ooo1-:0)?!;"/"o1- 

Explanation:

 Start with initial stack as input number 99+0 Push 18 and 0 to the top of the stack {:} Shift the stack to the left (wraps), copy the top value, and shift it back to the left (i.e. copy bottom of stack to the top) 1= Check to see if the top of the stack is equal to 1, pushes 1 for true, 0 for false ?. If top of stack is zero, skip the ., otherwise jumps to x,y coordinates on top of stack (18,0). This skips the next 8 instructions ~~ Pop the top 2 values from the stack (if they're not popped by the jump) " " Push the string literal " " onto the stack oo Pop the top two values of stack and output them as characters :2, Copy top value of stack, ad divide by 2 :1%- Since ><> uses float division, and doesn't have >= notation, remove the decimal part (if exists) v Redirect pointer down / Redirect pointer left :0) Copy top of stack, and see if its greater than 0 (1 for true, 0 for false) ?!v If top of stack is non-zero, then ! is executed, which skips the next instruction (redirect), otherwise, code is redirected "____" Push the literal "____" to the stack oooo Pop the top four values of stack and output them as characters 1- Decrement the top of the stack by 1 !/ Ignore the redirect action. When the loop gets to 0, it goes to next line, and gets redirected to the left. ~ Pops the top of the stack (0 counter) 42 Pushes 4 and 2 to the stack {:} As before, copies the bottom of the stack to the top 1=?. Also as before, if the initial value is 1, jump to (2,4) (skipping next 4 instructions ~~ Pop 2 values from stack if these instructions haven't been skipped ao Push 10 onto the stack and output it as a character (LF) "/ "oo Push the literal "/ " onto the stack and output it :// Copies the top of the stack then redirects to the line below, which then redirects to the left :0) Copies top of the stack and compares if its greater than 0 ?!v If it is, redirect to next line "\"o Push "\" to stack, then output it as a character 1- Decrement top value of stack :0)?!v If loop is not greater than 0, redirect to next line Either mode of redirect will loop to the left, and (potentially) skip the far right redirect because of the ! ao Push 10 to stack and output it as a character (LF) "/"o~ Push "/" to stack, then output it as a character. Pop top value of stack (the 0 from previous loop) v Redirects to next line, which then redirects to the right :0)?!; If the top of the stack is not greater than 0, terminate (;) "\__" Pushes "\__" to the stack ooo Outputs top 3 stack values as characters ("__\") 1- Decrement top of stack by 1 :0)?!; If the top of the stack is not greater than 0, terminate (;) "/"o Push "/" to top of stack then output it as a character 1- Decrement top of stack by 1 !\ Ignore the redirect 

><> (Fish), 220 bytes

Far from golfed, but it is my first fish program so far >_< The requirement to not have a blank first line for 1 triangle doubled the size of the program.

:1(?;:2,:1%-ff+3+1{:}1=?.~~" "oov "____"oooo1-  !\:0)?!v :oo" /" oa ~~.?=1}:{28~  /  -1ooo" /"v!?)0:-1o"\"v!?)0:  /!  v~ o"/"oa/!  / !\:0)?!;"\__"ooo1-:0)?!;"/"o1- 

Can test at http://fishlanguage.com/ (Int on initial stack for length)

Will get around to an explanation soon.

><> (Fish), 220 188 bytes

Far from golfed, but it is my first fish program so far >_< The requirement to not have a blank first line for 1 triangle doubled the size of the program.

99+0{:}1=?.~~" "oo:1(?;:2,:1%-v -1oooo"____"v!? )0:/! .?=1}:{2+aa~/:oo" /"oa~~ -1o"\"v!?)0:/!-1ooo" /"v!?)0: o"/"oa/! / v~ )?!;"\__"ooo1-:0)?!;"/"o1-!\:0 

Can test at http://fishlanguage.com/ (Int on initial stack for length)

Will get around to an explanation soon.