CL65 FLT CREW HAS RWY INCURSION AT LGA.

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PUSHED BACK AND GIVEN CLRNC TO TAXI TO RWY 4 VIA TXWY A; TXWY E; TXWY B. WE ENDED UP TAXIING VIA TXWY A; R ON TXWY E; L ON TXWY B; AND R ON TXWY D SHORT OF RWY 4; WITH THE NOSE OF THE ACFT OVER THE HOLD SHORT LINES FOR RWY 4. WE IMMEDIATELY CONTACTED TWR (WE WERE ALREADY TOLD TO MONITOR TWR) AND GAVE OUR POS. TWR CLRED US TO CROSS RWY 4 AT TXWY D; AND TAXI TO RWY 4 ON THE OTHER SIDE; WITH A DASH 8 HOLDING IN POS ON RWY 4. TAXIING VIA TXWY A ALONG THE RAMP (INNER) WITH TXWY B OFF THE R SIDE OF THE ACFT IS EASY UNTIL THE TRANSITION TO TXWY B USING TXWY E -- ESPECIALLY AT NIGHT IN RAIN. THE R TURN ON TXWY E AND SUBSEQUENT L TURN ON TXWY B LOOKS TO BE CUT AND DRY ON THE ARPT DIAGRAM; AS IT SHOWS TXWY B AS ITS OWN SEPARATE TXWY (SEPARATE FROM THE RAMP AREA) WHILE TXWY A IS THE 'INNER' RUNNING ALONG THE EDGE OF THE RAMP AREA. ONCE THE TURN WAS MADE ONTO TXWY B; IT APPEARED WE WERE STILL ON TXWY A; AS TXWY B IS MERELY ANOTHER YELLOW LINE ALONG THE EDGE OF THE RAMP ALONGSIDE OF TXWY A. IN THE RAIN AND DARKNESS IT WAS IMPOSSIBLE TO SEE THE TXWY A LINE ON OUR L. THE ILLUSION WAS THAT IN TAXIING I HAD MERELY MADE A L JOG OVER ON THE SAME LINE; AND WAS STILL ON TXWY A; AND TXWY B WAS STILL 1 TXWY OVER TO THE R. I ASKED THE FO I F HE THOUGHT I WAS ON IT (TXWY B) AND HE SAID NO IT WAS OVER TO THE R; AND SAID 'LET'S JUST USE THE NEXT R (TXWY D) TO GET OVER' I SHOULD HAVE STOPPED HERE; AND ASKED TWR; BUT CONTINUED TO TXWY D AND TURNED R. AS I MADE THE TURN; I SAW THE HOLD SHORT LINES FOR THE RWY AND APPLIED THE BRAKES. I TOLD THE FO TO TELL TWR; WHICH HE DID; AND WE WERE SUBSEQUENTLY CLRED ACROSS. MY ERROR: CONTINUING WHEN I FELT CONFUSED AND DISORIENTED. LOOKING AT THE ARPT DIAGRAM; ONE WOULD EXPECT THAT TXWY B AT THE SPOT I DESCRIBED IS A SEPARATE TXWY FROM THE RAMP AREA; AND THEREFORE EXPECT TO SEE BLUE LIGHTS ON THE L AND R. IN THE RAIN AT NIGHT IT LOOKS LIKE IT'S PART OF THE RAMP AND LOOKS LIKE IT IS TXWY A.