Most of them require installation of DCC and sound.

It depends on how electronically savvy you are, but most beginners would be better off paying someone to put DCC and sound into their engines instead of trying to do it themselves with no knowledge.  Even though there are good videos out there of how to do it, you may want for the first one or two have someone do it for you so you can at least start running a train instead of fooling around all night trying to figure out some task you're unfamiliar with and possibly damaging stuff in the process. It won't be cheap, but the job will be right, and you can immediately start operations.

Then you need to consider which DCC system to buy.  There are several out there now, all of which have pluses and minuses.  A lot of people start out with the Bachmann EZ command, like I did, but you'll find after a while that it is relatively limited compared to more advanced systems.  This is not a knock on the EZ command, it really is an ingenious creation for what it is, but it's really directed more to basic train set operation instead of more serious Model Railroading. I have to confess I have not looked, but there are certain to be multiple videos on YouTube about how to pick out a system.

 DCC operation has a learning curve involved, but there are lots of videos, and a bunch of us here on the forum will help you along the way if you need it.  I have to say that you're making the right choice, once you get into the swing of DCC and sound, there's nothing to compare.  Operating silent DC trains will in comparison feel like pushing a corpse around the track..

Steam? Diesel?

If steam, I can offer the following points.

Bachmann has the USRA Pacific and Mikado available right now from retailers.  However, the new ones right now don't come installed DCC/sound decoders. Some of the road names in the last run did, and they may still be around at various outlets.

Bachmann used to offer several other Spectrum series locomotives larger than those, and eBay is always rich with them if you look for them.  The light and heavy mountains, the light 2-10-2, there was even a 2-6-6-2 at one point.  However, the greater majority of these are going to be DC only locomotives, and if you want a big mama Jama engine, you're probably going to have to get accustomed to the idea of getting aftermarket DCC and sound into them. 

I could be wrong of course,  but the nature of your question makes me believe that you are a beginner in Model Railroading.  I need to ask the question, what is the minimum radius curve on your layout? Because even though all of the engines listed above will somehow get through an 18 inch radius curve, they will all look ridiculous on it hanging over front and back, not to mention clearance problems with items close to the outside of the curve. 

Plus, even though some of those engines may be called "DCC ready," they really are not, they require selection of the right speaker, installing the speaker so it doesn't rattle, soldering the decoder wires to the speaker, and making sure that the plug for the decoder into the motherboard is the right plug, they evolved over time and some of these early engines may not have the right plug for the modern decoder.  In other words, you may want to engage a professional to do this work for you.  They aren't cheap, but your engine comes back operating correctly without mistakes that beginners can make that can damage the decoder or the locomotive.

I'm sorry if I seem like I'm throwing a wet blanket over what you want, but those are the facts about these older engines and trying to get them up-to-date.

Two controllers being out of phase is a good enough reason to not try to use them together on the same track.

You can do that.  Then all the different horns will be blowing at the same time and sounding ridiculous together. 

I'm not certain but I wouldn't risk it until someone knows more says it is ok

Kervin,  since we're in the Bach Man's house, I'm reluctant to recommend different brands of equipment here.  Bachmann offers an advanced system, there are people on the forum that use it,  and I'm sure they would be happy to discuss it with you.  It's not the system I use, but when I got it almost 20 years ago I wasn't aware of the bachmann system.  There are two other systems I am immediately aware of, I'm sure there are YouTube videos that can help you select your system.

 I spent a long time reading online about this one and that one, and finally made my choice based on what a couple of people told me.  I still have it. I have never looked back. I have been very happy running DCC operations since around 2008 or so.  If you send me a personal message, I'll tell you in detail about it, but I'm reluctant to do it out here in the main forum.

I won't be able to offer you any help in actually consisting multiple diesel engines since all I run is steam locomotives, I never have and never will do otherwise.  A couple of times I ran double headers on a steep grade on my layout, but I had another operator present, and each locomotive was controlled by a separate engineer, just like the real ones were. To be clear, you have to have separate cabs for each locomotive. Doing that, both engineers have to be extremely careful about proper speed control between the engines. The lead locomotive always needs to be a molecule faster than the second one or they will bunch up the slack and fight each other . Seperate control of prototype helper engines was a major complication in the days of steam and was one contributing factor to their end.  And a major source of fun on a model railroad.

One would need a more advanced system beyond the EZ command to truly "consist" locomotives to operate as a unit.  I suppose it's possible to run multiple engines at once with the EZ command, but they would have to be perfectly speed matched to not play tug-of-war with each other.  "Consisting" engines to make only the lead engine in charge and be the only one making certain sounds is an advanced feature on Advanced DCC systems.  Likewise with speed matching. 

I'm speaking about the basic EZ command, the plus version may have more advanced features. I'm not familiar with.

Questions keep popping up on the forum showing that limitations and purpose of the EZ command are not well understood.  You have to think of the EZ command as basically being the DCC version of the old train "transformer."  It is really directed to the train set market,  very good at running one DCC locomotive at a time, delivering basic sound control, and being able to address different locomotives, which are really intended to be held stationary until selected.  Beyond that, you need more selective and specifically directed control to simultaneous operation of individual units.  This is not a flaw of the EZ command, Bachmann really hit a home run with its creation, making DCC train sets an affordable possibility.

But they are not intended for more advanced model railroad operations.

Why do you want to do that? Run two Trains at once?  The EZ command is not really designed for that, that's what more advanced systems are made for, to have two handheld cabs independently running two Trains through one power connection with the track. The EZ command "plus" might be a different story though.

I'm no electrician, but I think that if you put two EZ commands feeding power to the same track, I'm very certain you're going to have electrical issues, I'll let someone more astute elucidate, but my sense says no that's not possible without damage.

It's not really "ready".  That is a misleading statement.  You have to choose the correct speakers and install them and solder leads to them.  You may or may not need a wiring harness.

There are multiple decoders available that would work, but the Bach Man's house isn't the place to discuss other brands of products besides his.  I have my own preferences and have had professionals install DCC and at least 30 locomotives. If you want to hear about it in detail, send me a PM.

YouTube is certainly a place to find numerous videos to help you.

You can call the service department at any of the number of decoder manufacturers to get help also.

I haven't looked, but there's probably a parts diagram for this engine of the Bachmann online store that would help you disassemble things.  However, you are taking the tender apart, not the engine, for decoder installation.

There are people more knowledgeable than I on this forum, but I believe that the black road bed and gray road bed track can be used together, if you don't mind the contrasting road bed color.

The thing about the black road bed track is that the rail is steel and will very readily rust, so you have to be very assiduous about keeping it clean.  If it was me, I'd use that black road bed track temporarily until it can replaced with the gray road bed nickel silver rail which is a lot easier to deal with over time .

I need to eventually take that binding 4-6-0 apart and try to fix it. Too much other stuff getting in the way of Model Railroading right now.

Call the Bachmann service department

You get a look at that video yet?  I haven't yet but it appeared to be one that addresses your issue. 

Of course, with YouTube videos, you have to remember that there's 1 million people out there that think they are experts on everything, talk too much,  and ramble on and on without ever getting to the point. With or without poorly aimed and wobbly videos.  So you have to hunt some to find the right one, but I know that a good one is there.

Something is letting that driver with the gear push-up tighter against the worm gear when the engines put on the track.  It sounds to me like a shim is needed somewhere.

There's a Bachmann 4-6-0 that belongs to my friend at my house right now with the exact same problem. I've not been brave enough to take it apart yet so I'm waiting to see what you find and do.  But I am 99% convinced that something is allowing too much vertical play in the axle that lets it move up too tightly against the worm when the weight of the engine move the axle vertically up.

You should take the 4-4-0 to a layout with a more advanced DCC system that can manipulate cv values, and do a cv8=8 reset.  This cannot be done on your EZ command.  The owner of this layout should know how to do a cv8 reset.

You should know how to do this anyway if you're going to be doing DCC Model Railroading, after you acquire a more advanced DCC system, which you really should do if you're going to pursue more involved operations.

https://youtu.be/pzGIz7e8RWk?si=KIpAdor7fXYKW0aU