I would use the VR Lapland cars from Finland. The cars are bilevel and are designed to run on OHE (overhead electric) routes too so it fits in tunnels fairly easily. They're similar in height to the Superliner cars so transitions to engines won't look awkward. In fact these actually will match the ALC-42 super well in my opinion.
The top photo is a VR Sleeper and the bottom is a VR Coach:
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Simply Railways did a review of a night train featuring these cars. They certainly could be adequate superliner replacements assuming they could be repurposed for American railways at a reasonable price.
that said, Siemens already has such a huge presence in America that it makes sense to go with whatever they offer.