Express 505 was awesome. i loved going WTC by Express 505 &
coming back too. I was probably in Primary 5 then.
I think it followed 21's route out of Pasir Ris Int. Then along
Tampines Ave 4, turn right to Avenue 5, left to Avenue 10, Bedok
North Rd Ext, PIE, CTE Jln Bukit Merah, Alexandra Rd, Pasir Panjang
Rd, Lower Delta Rd, and back to Jln Bukit Merah/CTE/PIE.
I remember Scania N113 CAC on 505.
I miss all the former Express buses like 501, 503, 505, 507,
511 and 512 in the early 2000s. Those were the heydays. 
They should re-introduce them back.
501 - Sengkang to Marina Sq. Use the old route of 502 from
Orchard to Sengkang.
505 - Punggol to Clementi via KPE, ECP, MBS, Harbourfront
and Pasir Panjang (West Coast Highway).
507 - Hougang Central via Serangoon, KPE, ECP, MBS, Shenton Way,
Raffles Place, Orchard Rd and loop at Scotts Rd to return to
Hougang Central.
511 - Replace 97e. (Peak Hour service only)
512 - Replace 10e (Peak Hour service only)
513 - Repace 14e (Peak Hour service only)
515 - Repalce 196e (Peak Hour service only)
516 - Replace 174e (Peak Hour service only)
510 - Merge with 74e and 151e and amended to ply from Punggol,
Sengkang, Hougang, Serangoon North, CTE, PIE, Bukit Timah, Ngee Ann
Poly, NUS to Kent Ridge.
518 rename as 508.
502 and 506 remains the same.