Just curious, what is the high-speed route between Chicago and Indianapolis you are referring to? I know that there's the happenings of the SSL (this is not high-speed though) happening down to Dyer, so some of that way will be electrified.
The interim "higher-speed" option to Indy would likely utilize the SSL to Dyer (which works if the St. Charles Air Line Connector gets built), with higher speeds (110mph) south of there, employing the current Cardinal (former Hoosier State) Monon route.
In an HSRA video call with Amtrak's head of State-Supported Services, it was mentioned that a higher-speed run to Indy could be held up as "an example of what can be done".
A true high-speed service CHI-IND would likely make use of the current Monon sub (or an alignment close-by), based on conjectural routings. Some have shown a routing similar to the SSL to Dyer. HSRA has suggested converting a set of MED/SSL/CN tracks to high speed for intercity routes and airport express. Metra either had recently explored, or will be exploring, the cost of installing constant-tension catenary on the MED, something that would be necessary if HSR were to ever run on that line in the future.
HSRA has also suggested a connection to the CFE at Tolleston continuing to Wanatah, then heading south on a restored Monon ROW connecting to the current Cardinal routing at the town of Monon. In that scenario, Dyer and Rensselaer are dropped as stops and Gary and Valparaiso become new stops.
In the same video call, it was mentioned that Amtrak is still keeping its options open regarding a dedicated high-speed route out of Chicago to serve markets to the east. The Amtrak rep talked about taking the St. Charles Air Line Connector (if it ever gets built) onto MED/SSL/CN trackage to Grand Crossing, where trains would continue onto a rebuilt NYC ROW, currently being used by Commonwealth Edison for their high tension electrical towers. He then suggested trains would continue onto Porter into Michigan. It is conceivable that same NYC routing could accommodate a connection to the CFE, if it followed the route of the abandoned Ft. Wayne Secondary, which could be picked up around Buffington Harbor.
The only alignment I've seen that does not utilize the Monon is also from HSRA. High-speed trains would run to Champaign, with half heading southwest to Decatur, Springfield, and St. Louis, and the other half running east to Crawfordsville and Indy. Alon Levy posted a map in which Champaign would connect to Indianapolis via Lafayette.
HRSA's other idea would use the same high-speed tracks in Illinois, but instead turning southeast at Kankakee connecting to the Monon alignment at Lafayette on its way to Indy.
It's all just lines drawn on maps at the moment, but considering the distance between Chicago and Indy, as well as Indy's potential to become a vital new rail hub, long-term planning to go to higher-speed rail then onto high-speed rail should start now.
On the subject of the SSL, in a joint presentation before the Sandhouse Rail Group at Northwestern University
in February, 2023, Metra's Jim Derwinski and the SSL's Mike Noland discussed how the two railroads are working together for improvements on the MED line, including new bi-directional signaling from 11th Pl to 67th St, and speed improvements to increase MAS from 65mph to 79mph.
Noland presented the SSL's goal of eventually getting South Bend to Chicago service down to 90 minutes or less, making repeated references to the success enjoyed by the towns that are served by the Amtrak Hiawatha Service, with hopes that near identical travel times to South Bend would translate into similar economic benefits for the NW Indiana region.