“Don’t use electrical tape to hold wires together like I do”

There are many options today but the pictured connection is one of several valid wire to wire splice approaches including side by side and T splices.  The wires must be clean (electrical), wound as shown (mechanical).  A covering provides insulation, protection from the environment, and possibly additional mechanical support.  If the splice is pulled or subject to repetitive motion (flexed) it could loosen.  This would lead to loss of contact and oxidation.

At low voltage almost any tape would do for insulation.  The problem is most tapes do not age well because the adhesive drys out or oozes or the tape itself deteriorates.  Also, a failing splice on a high current line would get hot and affect the tape.

Two rules:

If you are going to use heat shrink or spaghetti, be sure to put the piece on the wire BEFORE you make the splice.

Never intentionally use Fahnestock clips for any purpose.

penn station posted:

...  If the splice is pulled or subject to repetitive motion (flexed) it could loosen. ...

this reminded me of another part of a wire splice that is often overlooked as a source of failure.  when removing insulation it is VERY important that the wire not be "nicked".  even a slight bite into the wire can result in a weak spot and will break if the splice is exposed to any motion, which luckily in most instances is not the case.

add to this some heat shrink insulation... typically just strong enough to support a broken connection... and you are looking at one tough electrical problem to find.

when i cannot find an accurate size wire stripper or NO-NIK cutters on smaller than 28 gauge, i will try to just nick the insulation enough to pull it off rather than to risk touching the conductor with a sharp edge.

I think there is a bit of traveling different tracks in connecting wires going on in this discussion.  GRJ does a lot of electronic work inside locos and his Hakko iron is hard to beat with dial up heat settings and then heat shrink for insulation. What works in layout wiring is often unsuitable inside a locomotive. I have used wire nuts, solder and crimp joints on layouts and find all more than adequate if matched to the situation. I am inclined to use a soldering gun on layout wiring but seldom use a gun when working on a locomotive especially if electronics are in it.  Wire nuts and crimp connectors work fine as a temporary connection in locos for testing purposes but once I verify all is well in my wiring scheme I want solder and heat shrink. Most of the time it makes a more compact connection and space is often more precious inside a locomotive than on a layout. I have no experience in outdoor model railroads but would guess that they require many of the techniques and strategies involved in marine wiring.  I had sailboats for years and making air/water tight connections was imperative.  I kept a pair of torches on board with soldering adapters as well as liquid tape and heat shrink. I also kept small copper swaging sleves that could be used for electrical work with my swaging tool (looks like a bolt cutter). Which I kept on board mainly for repairs to the standing rig which holds the mast up.  To make a weather proof swaged electrical connection I would paint liquid tape on the joint and slip heat shrink over it. Shrink it from the center outward before the liquid tape cures and excess would ooze out the ends. The resulting splice would be both air and water proof.         j

I don't usually solder inside an engine. Maybe if I have to make a wire longer for some reason and it easier to extend it with solder and heat shrink. But in general, it's too difficult to redo if you have to replace something and get the wires out of the way. I use the same tiny wire nuts that Lionel uses. I don't wrap anything with Scotch electrical tape because it turns gummy and gooey after awhile and it's not necessary if you twist the wire nut on properly. I use the tiny wire ties to neaten up the wires and prevent pinching when you put the shell back.

I solder and heatshrink things, they don't come apart after that.  I hate the wire nuts as they are too often found in the bottom of the locomotive when I remove the shell!