I am not referring to specs but to FRA definitions. According to the FRA SMART is Conventional Rail not heavy rail.

Source:
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Federal Railroad Administration

49 CFR Parts 209 and 211

[FRA Docket No. FRA-1999-5685, Notice No. 7]
RIN 2130-AB33

Statement of Agency Policy Concerning Jurisdiction Over the
Safety of Railroad Passenger Operations and Waivers Related to Shared
Use of the Tracks of the General Railroad System by Light Rail and
Conventional Equipment

AGENCY: Federal Railroad Administration (FRA), Department of
Transportation (DOT).
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Here's the relevant extract:
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Definition of ``Heavy Rail''
One commenter contends that FRA improperly defines the terms
``heavy rail'' and ``light rail'' in the proposed policy statement. The
commenter states that the term ``heavy'' has nothing to do with
crashworthiness or car weight, but rather applies to the construction
of the right-of-way, and suggests that it would be clearer to use the
terms ``rail rapid transit'' for what is incorrectly called heavy rail,
and ``urban electric transit'' for light rail.
Contrary to the commenter's statements, FRA's proposal properly
distinguished between the terms ``heavy rail'' and ``light rail.''
After observing that some current and planned passenger operations in
metropolitan areas are often referred to as ``light rail,'' FRA
indicated that the term usually refers to lightweight passenger cars
operating on rails in a right-of-way that is not separated from other
traffic, such

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as street railways and trolleys. 64 FR at 59049. FRA also stated that
``heavy rail'' generally refers to trains operating on rails that are
in separate rights-of-way from which all other vehicular traffic is
excluded, and observed that in transit terms, heavy rail is also known
as ``rapid rail,'' ``subway,'' or ``elevated railway.'' FRA noted that
conventional rail equipment such as that used by freight railroads,
Amtrak, and many commuter railroads is different from, and considerably
heavier and structurally stronger than either light or heavy rail
equipment, as those terms are used in the transit industry. FRA advised
that although this equipment is sometimes referred to as ``heavy''
rail, it would use the term ``conventional'' to avoid confusion between
the different ways ``heavy'' is used in the transit and general
railroad communities.