I can't spot any difference. I heard somewhere that the engine
sound was different.
But i watched videos of both from YouTube and couldn't spot any
difference.
EXTERIOR
1) C651 looks cleaner and whiter. Green-tinted glass
windows.
2) For the trip number on the train, C151 - plastic roller and
C651 - green flip dot (similar to TIBS bus top display)
3) C151 and C651 have diff. door opening sound.
4) SMRT logo at the sides: C151 mrt logo is about halfway
between car edge and door. C651 mrt logo is closer to the car edge.
i.e. further away from the door than c151
INTERIOR
1) Look for the number. The 2nd no. for C151 is either 0 or 1.
For c651, it's 2 (being 2nd-generation trains).
2) Glass windows. For C651 Siemens trains, it's made in Germany,
so the glass is made there too. At the lower corners, you should be
able to see the word 'Glastroch' imprinted on the glass. Greenish
tint (as mentioned above).
3) Aircon vent. C151 uses aluminium( or some metal?) and it's
stained due to ageing. C651 uses PVC (or plastic, I think).
4) For C651 doors, the windows are spaced further apart
than in C151 doors.
5) Door handles inside. C151 have door handles screwed onto the
door. C651 doesn't have anything screwed on the door. Just an
indent for door handle.
6) At the bottom near the door, C651 has black flooring strips.
C151 does not.
7) Side glass panels. C151 is flat-top. C651 is triangular at
the top, and it extends higher up.
8) Top lights. The C151 has squarish little boxes on the glass
cover while C651 looks plain.
9) Seats. At seat edges just before doors, C651 has an
additional vertical metal strip (in addition to the bottom
horizontal one) which is not found in C151.
10) Metal grab poles. C151 metal poles have round larger bases
at the tips not found in C651.
11) Panel for ads: C151 panel is more rounded at the edges
(and has more curves and more 'shapely') but C651 is more
angular and 'sharp'. Not to mention C651 top ad panels don't
'protrude out' as much as C151.
12) MRT has two glass panels for one window section right?
Observe the centre divider of the two panels. C151 has a
'connector' screwed onto the centre divider. C651 has nothing
screwed on; just a clean vertical line separating the two.
ENGINE
1) C151 - chopper control and C651- GTO. Different sounds. Just
take it and listen, especially during acceleration /
deceleration.